diff --git a/library/Tiger/Theme.php b/library/Tiger/Theme.php
index b0e50a6..33a076d 100644
--- a/library/Tiger/Theme.php
+++ b/library/Tiger/Theme.php
@@ -231,14 +231,19 @@ public static function active()
      * @return array{slug:string,key:string,asset_base:string}
      * @throws RuntimeException when no theme with that slug is on disk
      */
-    public static function activate($slug)
+    public static function activate($slug, $makeDefault = true)
     {
         $d = self::_discovered($slug);
         $key  = (string) ($d['key'] ?? preg_replace('/^theme-/', '', $slug));
         $base = ((string) ($d['asset_base'] ?? '')) !== '' ? (string) $d['asset_base'] : '/_' . $key;
-        (new Tiger_Model_Config())->set(Tiger_Model_Config::SCOPE_GLOBAL, '', 'tiger.theme', $key);
+        // Multiple themes can be active at once (their assets published, their layouts available); only
+        // ONE is the DEFAULT site theme (`tiger.theme`). Setting the default is opt-in, so activating a
+        // theme never hijacks the site — the Module manager's "make default" checkbox drives $makeDefault.
+        if ($makeDefault) {
+            (new Tiger_Model_Config())->set(Tiger_Model_Config::SCOPE_GLOBAL, '', 'tiger.theme', $key);
+        }
         self::_linkAssets($slug, $base, (string) ($d['area'] ?? 'app'));
-        return ['slug' => $slug, 'key' => $key, 'asset_base' => $base];
+        return ['slug' => $slug, 'key' => $key, 'asset_base' => $base, 'default' => (bool) $makeDefault];
     }
 
     /**
diff --git a/modules/system/controllers/ModulesController.php b/modules/system/controllers/ModulesController.php
index f78a0ad..7a1daa6 100644
--- a/modules/system/controllers/ModulesController.php
+++ b/modules/system/controllers/ModulesController.php
@@ -35,8 +35,10 @@ public function indexAction()
         foreach (Tiger_Module_Discovery::all() as $slug => $m) {
             $row     = $installed[$slug] ?? null;
             $isTheme = ($m['type'] ?? 'module') === 'theme';
-            // A theme's active state is the tiger.theme config (its KEY, one per scope); a module's is its flag.
-            $active  = $isTheme ? ($activeTheme === ($m['key'] ?? $slug)) : ($row ? ((int) $row->active === 1) : true);
+            // Active is the module FLAG for everything now, themes included — multiple themes can be
+            // active at once. WHICH theme is the DEFAULT site theme is the separate `tiger.theme` config.
+            $active    = $row ? ((int) $row->active === 1) : true;
+            $isDefault = $isTheme && $activeTheme !== '' && $activeTheme === (string) ($m['key'] ?? $slug);
             $source  = $row ? $row->source : ($m['area'] === 'core' ? 'bundled' : 'custom');
             // Taxonomy resolution (AUTHORING.md): the value STORED at install (retained from the source
             // listing/manifest) wins; else the live manifest that Discovery read; else its default. Read
@@ -49,6 +51,7 @@ public function indexAction()
             $m['protected'] = !empty($m['protected']) || in_array($slug, System_Service_Modules::PROTECTED, true);
             $modules[] = $m + [
                 'active'    => $active,
+                'is_default' => $isDefault,
                 'source'    => $source,
                 // Advisory: tested-version compat notice (never blocks) + who requires this module
                 // (drives the "required by X, Y — deactivate anyway?" confirm; empty for most).
diff --git a/modules/system/languages/en/system.php b/modules/system/languages/en/system.php
index 122cfbf..c493cc9 100644
--- a/modules/system/languages/en/system.php
+++ b/modules/system/languages/en/system.php
@@ -150,6 +150,8 @@
     'system.modules.source_addon'    => 'addon',
     'system.modules.status_active'   => 'Active',
     'system.modules.status_inactive' => 'Inactive',
+    'system.modules.default_theme'       => 'Default',
+    'system.modules.default_theme_title' => 'The default site theme — the chrome for the home page and any page that does not pick its own layout.',
     'system.modules.always_on'       => 'always on',
     'system.modules.deactivate'      => 'Deactivate',
     'system.modules.activate'        => 'Activate',
@@ -390,8 +392,9 @@
     'system.js.deactivate_ok' => 'Deactivate',
     'system.js.deactivate_required' => 'This module is required by %1$s to function. Deactivate anyway?',
     'system.js.deactivate_theme' => 'Deactivate the "%1$s" theme? The public site reverts to the default theme.',
-    'system.js.activate_theme_multi_title' => 'Multiple active themes',
-    'system.js.activate_theme_multi' => 'Another theme is already active. Activating more than one theme at a time can have unpredictable results — unless the theme is designed to work alongside others. Activate anyway?',
+    'system.js.activate_theme_title' => 'Activate theme',
+    'system.js.activate_theme_body' => 'Activate the "%s" theme? Its layouts become available to your pages, and any other active themes stay active.',
+    'system.js.activate_theme_make_default' => 'Make "%s" the default site theme',
     'system.js.deactivate_confirm' => 'Deactivate "%1$s"? Its routes stop working until you reactivate it. Nothing is deleted.',
     'system.js.confirm_install_title' => 'Install module',
     'system.js.confirm_install_body' => 'This module’s code will run in your app once active.',
diff --git a/modules/system/services/Modules.php b/modules/system/services/Modules.php
index 0f1afee..ce606b1 100644
--- a/modules/system/services/Modules.php
+++ b/modules/system/services/Modules.php
@@ -104,13 +104,9 @@ public function delete(array $params): void
         }
     }
 
-    /** Whether a module is currently active — a theme by its `tiger.theme` config, else its registry flag. */
+    /** Whether a module is currently active — the registry flag, themes included (multiple can be active). */
     private function _isModuleActive(string $slug, array $d): bool
     {
-        if (($d['type'] ?? '') === 'theme') {
-            $active = (string) (new Tiger_Model_Config())->get(Tiger_Model_Config::SCOPE_GLOBAL, '', 'tiger.theme');
-            return $active === (string) ($d['key'] ?? $slug);
-        }
         $row = (new Tiger_Model_Module())->bySlug($slug);
         return $row ? ((int) $row->active === 1) : true;
     }
@@ -172,7 +168,7 @@ protected function _toggle(array $params, $on): void
             // Themes activate differently (THEMES.md §5a): not the module.active flag, but the
             // `tiger.theme` config (one active per scope) + the asset-base symlink. No build/deploy.
             if (($d['type'] ?? 'module') === 'theme') {
-                $this->_toggleTheme($slug, $d, $on);
+                $this->_toggleTheme($slug, $d, $on, $params);
                 return;
             }
 
@@ -198,19 +194,30 @@ protected function _toggle(array $params, $on): void
     }
 
     /**
-     * Activate/deactivate a THEME (THEMES.md §5a) — through the one authority, Tiger_Theme::activate()
-     * / deactivate(), which the headless installer calls too. Activation writes `tiger.theme` and links
-     * the theme's assets; deactivation clears the config back to the platform base theme.
+     * Activate/deactivate a THEME (THEMES.md §5a). Multiple themes can be active at once — activation is
+     * the module active FLAG plus publishing the theme's assets. Making it the DEFAULT site theme
+     * (`tiger.theme`) is opt-in: only when `make_default` is set (the Module manager's checkbox), so
+     * activating a theme never silently hijacks the site. Deactivation clears the flag and, if this was
+     * the default, `Tiger_Theme::deactivate` clears `tiger.theme` (home falls back to blank).
      *
-     * @param  string $slug the theme slug
-     * @param  array  $d     its discovery row (unused here; kept for the caller's signature)
-     * @param  bool   $on    activate (true) or deactivate (false)
+     * @param  string $slug   the theme slug
+     * @param  array  $d       its discovery row (name/version for the module row)
+     * @param  bool   $on      activate (true) or deactivate (false)
+     * @param  array  $params  the /api payload (reads `make_default`)
      * @return void
      */
-    protected function _toggleTheme($slug, array $d, $on): void
-    {
-        if ($on) { Tiger_Theme::activate($slug); }
-        else     { Tiger_Theme::deactivate($slug); }
+    protected function _toggleTheme($slug, array $d, $on, array $params = []): void
+    {
+        $model = new Tiger_Model_Module();
+        $meta  = ['name' => $d['name'] ?? $slug, 'version' => $d['version'] ?? null];
+        if ($on) {
+            $makeDefault = ((string) ($params['make_default'] ?? '')) === '1';
+            $model->setActive($slug, true, $meta);
+            Tiger_Theme::activate($slug, $makeDefault);
+        } else {
+            $model->setActive($slug, false, $meta);
+            Tiger_Theme::deactivate($slug);
+        }
         $this->_success(
             ['slug' => $slug, 'theme' => true, 'active' => (bool) $on],
             $on ? 'system.theme.activated' : 'system.theme.deactivated',
diff --git a/modules/system/views/scripts/modules/index.phtml b/modules/system/views/scripts/modules/index.phtml
index 522e849..e1de4a6 100644
--- a/modules/system/views/scripts/modules/index.phtml
+++ b/modules/system/views/scripts/modules/index.phtml
@@ -105,6 +105,7 @@ $typeIcon = static function ($t) {
                         <td data-order="<?= $m['active'] ? 1 : 0 ?>">
                             <?php if ($m['active']): ?><span class="badge text-bg-success"><?= $this->t('system.modules.status_active') ?></span>
                             <?php else: ?><span class="badge text-bg-secondary"><?= $this->t('system.modules.status_inactive') ?></span><?php endif; ?>
+                            <?php if (!empty($m['is_default'])): ?><span class="badge text-bg-primary ms-1" title="<?= $this->escape($this->t('system.modules.default_theme_title')) ?>"><i class="fa-solid fa-star me-1"></i><?= $this->t('system.modules.default_theme') ?></span><?php endif; ?>
                         </td>
                         <td class="text-end text-nowrap">
                             <?php if ($m['protected']): ?>
@@ -112,7 +113,7 @@ $typeIcon = static function ($t) {
                             <?php elseif ($m['active']): ?>
                                 <button type="button" class="btn btn-sm btn-outline-warning" data-mod-toggle="deactivate" data-slug="<?= $this->escape($m['slug']) ?>" data-theme="<?= (($m['type'] ?? '') === 'theme') ? 1 : 0 ?>" data-required-by="<?= $this->escape(implode(',', array_values($m['required_by'] ?? []))) ?>"><i class="fa-solid fa-pause me-1"></i><?= $this->t('system.modules.deactivate') ?></button>
                             <?php else: ?>
-                                <button type="button" class="btn btn-sm btn-outline-success" data-mod-toggle="activate" data-slug="<?= $this->escape($m['slug']) ?>" data-theme="<?= (($m['type'] ?? '') === 'theme') ? 1 : 0 ?>"><i class="fa-solid fa-play me-1"></i><?= $this->t('system.modules.activate') ?></button>
+                                <button type="button" class="btn btn-sm btn-outline-success" data-mod-toggle="activate" data-slug="<?= $this->escape($m['slug']) ?>" data-theme="<?= (($m['type'] ?? '') === 'theme') ? 1 : 0 ?>" data-name="<?= $this->escape($m['name']) ?>" data-default-exists="<?= ($this->activeTheme ?? '') !== '' ? 1 : 0 ?>"><i class="fa-solid fa-play me-1"></i><?= $this->t('system.modules.activate') ?></button>
                             <?php endif; ?>
                             <?php $deletable = !$m['protected'] && (($m['area'] ?? '') !== 'core') && !$m['active']; ?>
                             <?php if ($deletable): ?>
@@ -166,8 +167,9 @@ $this->i18n([
     'deactivateRequired' => 'system.js.deactivate_required',
     'deactivateTheme' => 'system.js.deactivate_theme',
     'deactivateConfirm' => 'system.js.deactivate_confirm',
-    'activateThemeMultiTitle' => 'system.js.activate_theme_multi_title',
-    'activateThemeMulti' => 'system.js.activate_theme_multi',
+    'activateThemeTitle' => 'system.js.activate_theme_title',
+    'activateThemeBody' => 'system.js.activate_theme_body',
+    'activateThemeMakeDefault' => 'system.js.activate_theme_make_default',
 ]);
 ?>
 <script>
@@ -216,8 +218,8 @@ document.addEventListener('DOMContentLoaded', function () {
                     .then(function (r) { return r.json(); });
             });
         }
-        function run() {
-            send().then(function (res) {
+        function run(extra) {
+            send(extra).then(function (res) {
                 if (res.result === 1) { window.location.reload(); return; }
                 if (res.data && res.data.conflicts && res.data.conflicts.length) {
                     var names = res.data.conflicts.map(function (c) { return '"' + (c.name || c.slug) + '"'; }).join(', ');
@@ -228,7 +230,7 @@ document.addEventListener('DOMContentLoaded', function () {
                         variant: 'primary'
                     }).then(function (ok) {
                         if (!ok) { return; }
-                        send({ confirm: '1' }).then(function (r2) {
+                        send(Object.assign({ confirm: '1' }, extra || {})).then(function (r2) {
                             if (r2.result === 1) { window.location.reload(); return; }
                             (r2.messages || []).forEach(function (m) { TigerDOM.toast(m.message, { type: m.class || 'error' }); });
                         }).catch(function () {});
@@ -257,23 +259,24 @@ document.addEventListener('DOMContentLoaded', function () {
             return;
         }
 
-        /* Activating a theme while another theme is already active is ALLOWED (a theme may be built to
-           coexist), but it can be unpredictable — surface a modal NOTICE first, then proceed on OK.
-           Active themes are the rows showing a Deactivate button tagged data-theme="1". */
+        /* Activating a THEME never deactivates another — multiple themes can be active at once (their
+           layouts stay available for pages to use). Offer a checkbox to also make this the DEFAULT site
+           theme; the default only switches when it's ticked. When nothing is the default yet, pre-tick it
+           so the first activated theme becomes the site theme. */
         if (isTheme) {
-            var otherActiveThemes = Array.prototype.filter.call(
-                document.querySelectorAll('[data-mod-toggle="deactivate"][data-theme="1"]'),
-                function (b) { return b.getAttribute('data-slug') !== slug; }
-            );
-            if (otherActiveThemes.length) {
-                TigerModal.confirm({
-                    title: Tiger.t('activateThemeMultiTitle'),
-                    body: Tiger.t('activateThemeMulti'),
-                    confirmLabel: Tiger.t('activateOk'),
-                    variant: 'primary'
-                }).then(function (ok) { if (ok) { run(); } });
-                return;
-            }
+            var themeName    = btn.getAttribute('data-name') || slug;
+            var noDefaultYet = btn.getAttribute('data-default-exists') === '0';
+            TigerModal.confirm({
+                title: Tiger.t('activateThemeTitle'),
+                body: Tiger.t('activateThemeBody', themeName),
+                confirmLabel: Tiger.t('activateOk'),
+                variant: 'primary',
+                checkbox: { label: Tiger.t('activateThemeMakeDefault', themeName), checked: noDefaultYet }
+            }).then(function (res) {
+                if (!res || !res.confirmed) { return; }
+                run(res.checked ? { make_default: '1' } : {});
+            });
+            return;
         }
         run();
     }
diff --git a/tests/Integration/System/ModulesServiceThemeTest.php b/tests/Integration/System/ModulesServiceThemeTest.php
index d140d3c..c1d772f 100644
--- a/tests/Integration/System/ModulesServiceThemeTest.php
+++ b/tests/Integration/System/ModulesServiceThemeTest.php
@@ -9,14 +9,16 @@
 use System_Service_Modules;
 use Tiger\Tests\Support\IntegrationTestCase;
 use Tiger_Model_Config;
+use Tiger_Model_Module;
 
 /**
  * System_Service_Modules — the THEME-toggle path ModulesServiceExtraTest flagged as unreachable ("needs
  * a type:theme module on disk"). We plant a throwaway `theme-<x>` module (a bare `theme.json` — no
  * Bootstrap, no assets/, no migrations/) under the harness app modules dir so live discovery sees a
- * `type:theme` row. Activating a theme is NOT the `module.active` flag: it writes `tiger.theme = <key>`
- * to the config tier (global scope) and — because the fixture ships no assets/ — the asset-symlink step
- * short-circuits. Deactivating clears the config back to the base theme.
+ * `type:theme` row. Multiple themes can be active at once: activating sets the module active FLAG (and,
+ * because the fixture ships no assets/, the asset-symlink step short-circuits). Which theme is the
+ * DEFAULT site theme is the separate `tiger.theme` config, written only when `make_default` is set.
+ * Deactivating clears the flag and, if this was the default, clears the config back to the base theme.
  *
  * All config writes are the DB tier (rolled back per test); the fixture dir is removed in tearDown.
  */
@@ -62,27 +64,40 @@ private function activeTheme(): string
         return (string) (new Tiger_Model_Config())->get(Tiger_Model_Config::SCOPE_GLOBAL, '', 'tiger.theme');
     }
 
+    private function moduleActive(string $slug): bool
+    {
+        $row = (new Tiger_Model_Module())->bySlug($slug);
+        return $row ? ((int) $row->active === 1) : false;
+    }
+
     #[Test]
-    public function activating_a_theme_writes_the_tiger_theme_config_not_a_module_flag(): void
+    public function activating_a_theme_sets_the_flag_and_makes_it_default_only_when_asked(): void
     {
         $this->loginAs('superadmin');
-        $res = $this->dispatch(['action' => 'activate', 'slug' => self::SLUG]);
 
+        // Plain activate: the module active FLAG goes on (multiple themes can be active), but the
+        // DEFAULT site theme is NOT changed — activating a theme never silently hijacks the site.
+        $res = $this->dispatch(['action' => 'activate', 'slug' => self::SLUG]);
         $this->assertSame(1, (int) $res->result, $this->messages($res));
         $this->assertStringContainsString('activated', $this->messages($res));
         $this->assertTrue((bool) $res->data['theme'], 'the response marks this as a theme activation');
         $this->assertTrue((bool) $res->data['active']);
         $this->assertSame('/system/modules', $res->redirect);
-        // The single config write: the active theme KEY at global scope.
-        $this->assertSame(self::KEY, $this->activeTheme(), 'tiger.theme now holds the theme key');
+        $this->assertTrue($this->moduleActive(self::SLUG), 'the module active flag is set');
+        $this->assertSame('', $this->activeTheme(), 'activating alone does not change the default site theme');
+
+        // Activate WITH make_default -> it becomes the default site theme (tiger.theme).
+        $this->dispatch(['action' => 'activate', 'slug' => self::SLUG, 'make_default' => '1']);
+        $this->assertSame(self::KEY, $this->activeTheme(), 'make_default writes tiger.theme');
     }
 
     #[Test]
-    public function deactivating_the_active_theme_clears_it_back_to_the_base(): void
+    public function deactivating_the_default_theme_clears_the_flag_and_the_default(): void
     {
         $this->loginAs('superadmin');
-        // Activate first, then deactivate — the deactivate arm only clears when the key currently matches.
-        $this->dispatch(['action' => 'activate', 'slug' => self::SLUG]);
+        // Activate as the default, then deactivate — the flag clears and, because it was the default,
+        // tiger.theme reverts to the base theme.
+        $this->dispatch(['action' => 'activate', 'slug' => self::SLUG, 'make_default' => '1']);
         $this->assertSame(self::KEY, $this->activeTheme());
 
         $res = $this->dispatch(['action' => 'deactivate', 'slug' => self::SLUG]);
@@ -90,7 +105,8 @@ public function deactivating_the_active_theme_clears_it_back_to_the_base(): void
         $this->assertStringContainsString('deactivated', $this->messages($res));
         $this->assertTrue((bool) $res->data['theme']);
         $this->assertFalse((bool) $res->data['active']);
-        $this->assertSame('', $this->activeTheme(), 'the config reverts to the base theme');
+        $this->assertFalse($this->moduleActive(self::SLUG), 'the module flag is cleared');
+        $this->assertSame('', $this->activeTheme(), 'the default reverts to the base theme');
     }
 
     #[Test]
diff --git a/tests/Integration/Theme/ThemeActivateTest.php b/tests/Integration/Theme/ThemeActivateTest.php
index fe1f0c9..b5f3709 100644
--- a/tests/Integration/Theme/ThemeActivateTest.php
+++ b/tests/Integration/Theme/ThemeActivateTest.php
@@ -63,8 +63,8 @@ private function activeTheme(): string
     public function activate_writes_the_config_key_and_links_the_assets(): void
     {
         $out = Tiger_Theme::activate(self::SLUG);
-        $this->assertSame(['slug' => self::SLUG, 'key' => self::KEY, 'asset_base' => '/_' . self::KEY], $out);
-        $this->assertSame(self::KEY, $this->activeTheme());
+        $this->assertSame(['slug' => self::SLUG, 'key' => self::KEY, 'asset_base' => '/_' . self::KEY, 'default' => true], $out);
+        $this->assertSame(self::KEY, $this->activeTheme(), 'activate() defaults to making it the default site theme');
         $this->assertTrue(is_dir($this->link), 'assets reachable under public/_<key>');
         $this->assertFileExists($this->link . '/css/q9z.css');
     }
diff --git a/themes/puma/assets/js/tiger.modal.js b/themes/puma/assets/js/tiger.modal.js
index 8f655d8..cf508c6 100644
--- a/themes/puma/assets/js/tiger.modal.js
+++ b/themes/puma/assets/js/tiger.modal.js
@@ -7,6 +7,7 @@
  * so a control's "are you sure?" or "name this thing" moment looks like the rest of the product.
  *
  *   TigerModal.confirm({ title, body, confirmLabel, cancelLabel, variant }) -> Promise<boolean>
+ *   TigerModal.confirm({ ..., checkbox: { label, checked } }) -> Promise<{ confirmed, checked }>
  *   TigerModal.prompt({ title, label, value, placeholder, help, confirmLabel, variant }) -> Promise<string|null>
  *   TigerModal.alert({ title, body, confirmLabel, variant }) -> Promise<void>
  *
@@ -29,7 +30,7 @@
     'use strict';
     if (typeof window.bootstrap === 'undefined' || !window.bootstrap.Modal) { return; }
 
-    var el = null, modal = null, resolver = null, mode = 'confirm';
+    var el = null, modal = null, resolver = null, mode = 'confirm', hasCheckbox = false;
 
     function q(sel) { return el.querySelector('[data-tm="' + sel + '"]'); }
 
@@ -57,6 +58,10 @@
           +       '<input type="text" class="form-control" data-tm="input" autocomplete="off">'
           +       '<div class="form-text" data-tm="help"></div>'
           +     '</div>'
+          +     '<div class="form-check d-none" data-tm="checkwrap">'
+          +       '<input class="form-check-input" type="checkbox" data-tm="check" id="tm-check">'
+          +       '<label class="form-check-label" for="tm-check" data-tm="checklabel"></label>'
+          +     '</div>'
           +   '</div>'
           +   '<div class="modal-footer">'
           +     '<button type="button" class="btn btn-light" data-tm="cancel" data-bs-dismiss="modal"></button>'
@@ -67,7 +72,7 @@
         modal = window.bootstrap.Modal.getOrCreateInstance(el);
 
         q('confirm').addEventListener('click', function () {
-            settle(mode === 'prompt' ? q('input').value : true);
+            settle(mode === 'prompt' ? q('input').value : (hasCheckbox ? { confirmed: true, checked: q('check').checked } : true));
             modal.hide();
         });
         q('input').addEventListener('keydown', function (e) {
@@ -75,7 +80,7 @@
         });
         // Any dismissal that isn't a confirm (Cancel, ✕, Esc, backdrop) resolves the cancel value.
         el.addEventListener('hidden.bs.modal', function () {
-            settle(mode === 'prompt' ? null : (mode === 'alert' ? undefined : false));
+            settle(mode === 'prompt' ? null : (mode === 'alert' ? undefined : (hasCheckbox ? { confirmed: false, checked: false } : false)));
         });
         el.addEventListener('shown.bs.modal', function () {
             if (mode === 'prompt') { var i = q('input'); i.focus(); i.select(); }
@@ -117,6 +122,15 @@
             field.classList.add('d-none');
         }
 
+        // Optional checkbox (confirm only): { label, checked }. With it, confirm resolves
+        // { confirmed, checked } instead of a bare boolean; without it, behaviour is unchanged.
+        hasCheckbox = (kind === 'confirm' && opts.checkbox && typeof opts.checkbox === 'object');
+        q('checkwrap').classList.toggle('d-none', !hasCheckbox);
+        if (hasCheckbox) {
+            q('checklabel').textContent = opts.checkbox.label || '';
+            q('check').checked = !!opts.checkbox.checked;
+        }
+
         return new Promise(function (resolve) { resolver = resolve; modal.show(); });
     }