diff --git a/87-ClaudeCode/03-claude-tetris/CLAUDE.md b/87-ClaudeCode/03-claude-tetris/CLAUDE.md
index dc6a082..604dd0a 100644
--- a/87-ClaudeCode/03-claude-tetris/CLAUDE.md
+++ b/87-ClaudeCode/03-claude-tetris/CLAUDE.md
@@ -20,4 +20,5 @@ All logic lives in a single script, [game.js](game.js), loaded by [index.html](i
 - **Piece lifecycle**: `lockPiece()` → `merge()` → `clearLines()` (updates score/level/`dropInterval`) → `spawn()` (promotes `next`; calls `endGame()` if the spawn position collides).
 - **Input** is one `keydown` handler using `e.code`. Rotation (`tryRotate`) is clockwise only, with simple horizontal wall kicks `[0, -1, 1, -2, 2]`.
 - **Scoring**: line clears use `LINE_SCORES × level`; soft drop gives +1 per cell and hard drop +2 per cell. Level is `floor(lines / 10) + 1`, and `dropInterval = max(100, 1000 - (level-1) * 90)` ms.
+- **Theming**: dark is the default. Colors live in CSS custom properties in [style.css](style.css) (`:root` = dark, `:root[data-theme="light"]` = light). The `#theme-toggle` button (ids `theme-toggle`, `theme-toggle-icon`, `theme-toggle-text`) calls `applyTheme()` in `game.js`, which sets `data-theme` on `<html>`, swaps `palette` between `COLORS` and `LIGHT_COLORS`, caches the `--grid` value into `gridColor` (canvas can't read CSS), and repaints via `draw()`/`drawNext()` (the loop is stopped while paused/game over). The choice is persisted in `localStorage` under `tetris-theme`. The button is blurred after click so Space doesn't re-trigger it. Any new color should be a CSS variable, not a hard-coded value.
 - **Rendering**: `draw()` repaints grid, board, ghost piece (via `ghostY()`, alpha 0.2) and current piece. `drawNext()` renders into the separate preview canvas.
diff --git a/87-ClaudeCode/03-claude-tetris/README.md b/87-ClaudeCode/03-claude-tetris/README.md
index ae4c040..aaef837 100644
--- a/87-ClaudeCode/03-claude-tetris/README.md
+++ b/87-ClaudeCode/03-claude-tetris/README.md
@@ -42,6 +42,7 @@ A playable version of classic Tetris with all the mechanics you would expect:
 - Classic Tetris **scoring system** (100 / 300 / 500 / 800 multiplied by level).
 - **Levels** that increase every 10 lines and speed up the fall.
 - **Pause** and **Game Over** with a restart option.
+- **Light / dark theme toggle** (top-right button). Dark is the default; the choice is remembered in `localStorage`.
 
 ---
 
diff --git a/87-ClaudeCode/03-claude-tetris/game.js b/87-ClaudeCode/03-claude-tetris/game.js
index d08fa06..4366acb 100644
--- a/87-ClaudeCode/03-claude-tetris/game.js
+++ b/87-ClaudeCode/03-claude-tetris/game.js
@@ -15,6 +15,18 @@ const COLORS = [
   '#ffb74d', // L - orange
 ];
 
+// Slightly deeper tones so pieces and the ghost stay visible on a white board.
+const LIGHT_COLORS = [
+  null,
+  '#00acc1', // I
+  '#f9a825', // O
+  '#8e24aa', // T
+  '#43a047', // S
+  '#e53935', // Z
+  '#1e88e5', // J
+  '#fb8c00', // L
+];
+
 const PIECES = [
   null,
   [[0,0,0,0],[1,1,1,1],[0,0,0,0],[0,0,0,0]], // I
@@ -39,6 +51,13 @@ const overlay = document.getElementById('overlay');
 const overlayTitle = document.getElementById('overlay-title');
 const overlayScore = document.getElementById('overlay-score');
 const restartBtn = document.getElementById('restart-btn');
+const themeToggle = document.getElementById('theme-toggle');
+const themeToggleIcon = document.getElementById('theme-toggle-icon');
+const themeToggleText = document.getElementById('theme-toggle-text');
+
+const THEME_KEY = 'tetris-theme';
+let gridColor = '#22222e';
+let palette = COLORS;
 
 let board, current, next, score, lines, level, paused, gameOver, lastTime, dropAccum, dropInterval, animId;
 
@@ -158,7 +177,7 @@ function updateHUD() {
 
 function drawBlock(context, x, y, colorIndex, size, alpha) {
   if (!colorIndex) return;
-  const color = COLORS[colorIndex];
+  const color = palette[colorIndex];
   context.globalAlpha = alpha ?? 1;
   context.fillStyle = color;
   context.fillRect(x * size + 1, y * size + 1, size - 2, size - 2);
@@ -169,7 +188,7 @@ function drawBlock(context, x, y, colorIndex, size, alpha) {
 }
 
 function drawGrid() {
-  ctx.strokeStyle = '#22222e';
+  ctx.strokeStyle = gridColor;
   ctx.lineWidth = 0.5;
   for (let c = 1; c < COLS; c++) {
     ctx.beginPath();
@@ -301,4 +320,34 @@ document.addEventListener('keydown', e => {
 
 restartBtn.addEventListener('click', init);
 
+function loadTheme() {
+  try {
+    return localStorage.getItem(THEME_KEY) === 'light' ? 'light' : 'dark';
+  } catch {
+    return 'dark';
+  }
+}
+
+function applyTheme(theme) {
+  const isLight = theme === 'light';
+  document.documentElement.dataset.theme = theme;
+  palette = isLight ? LIGHT_COLORS : COLORS;
+  themeToggle.setAttribute('aria-pressed', String(isLight));
+  themeToggleIcon.textContent = isLight ? '☾' : '☀';
+  themeToggleText.textContent = isLight ? 'Dark' : 'Light';
+  // The canvas can't read CSS variables, so cache the grid color here.
+  gridColor = getComputedStyle(document.documentElement).getPropertyValue('--grid').trim() || gridColor;
+  // The loop is stopped while paused/game over, so repaint explicitly.
+  if (board) { draw(); drawNext(); }
+}
+
+themeToggle.addEventListener('click', () => {
+  const theme = document.documentElement.dataset.theme === 'light' ? 'dark' : 'light';
+  try { localStorage.setItem(THEME_KEY, theme); } catch { /* storage unavailable */ }
+  applyTheme(theme);
+  themeToggle.blur(); // keep Space/Enter from re-triggering the toggle
+});
+
+applyTheme(loadTheme());
+
 init();
diff --git a/87-ClaudeCode/03-claude-tetris/index.html b/87-ClaudeCode/03-claude-tetris/index.html
index 3e59aca..7a81f7c 100644
--- a/87-ClaudeCode/03-claude-tetris/index.html
+++ b/87-ClaudeCode/03-claude-tetris/index.html
@@ -7,6 +7,11 @@
   <link rel="stylesheet" href="style.css" />
 </head>
 <body>
+  <button id="theme-toggle" class="theme-toggle" type="button" aria-pressed="false" aria-label="Light theme">
+    <span id="theme-toggle-icon" aria-hidden="true">&#9728;</span>
+    <span id="theme-toggle-text" aria-hidden="true">Light</span>
+  </button>
+
   <div class="wrapper">
     <h1 class="title">TETRIS</h1>
 
diff --git a/87-ClaudeCode/03-claude-tetris/style.css b/87-ClaudeCode/03-claude-tetris/style.css
index 5765d27..7aa8577 100644
--- a/87-ClaudeCode/03-claude-tetris/style.css
+++ b/87-ClaudeCode/03-claude-tetris/style.css
@@ -1,3 +1,44 @@
+/* Dark is the default theme; light overrides apply under [data-theme="light"]. */
+:root {
+  color-scheme: dark;
+  --bg: #0f0f17;
+  --text: #e0e0e0;
+  --accent: #7aa2f7;
+  --accent-hover: #9bbdff;
+  --accent-contrast: #0f0f17;
+  --board-bg: #1a1a25;
+  --border: #2a2a3a;
+  --shadow: rgba(0, 0, 0, 0.6);
+  --label: #555570;
+  --muted: #888;
+  --kbd-bg: #22223a;
+  --kbd-border: #3a3a5a;
+  --kbd-text: #aaa;
+  --overlay-bg: rgba(10, 10, 20, 0.75);
+  --danger: #e57373;
+  --grid: #22222e;
+}
+
+:root[data-theme="light"] {
+  color-scheme: light;
+  --bg: #f1f3f9;
+  --text: #22243a;
+  --accent: #3b5fc0;
+  --accent-hover: #2f4ea1;
+  --accent-contrast: #ffffff;
+  --board-bg: #ffffff;
+  --border: #c5c9d9;
+  --shadow: rgba(40, 50, 90, 0.18);
+  --label: #6a6e88;
+  --muted: #555a70;
+  --kbd-bg: #e6e9f4;
+  --kbd-border: #b8bdd2;
+  --kbd-text: #3a3e58;
+  --overlay-bg: rgba(241, 243, 249, 0.8);
+  --danger: #c62828;
+  --grid: #e3e6f0;
+}
+
 *, *::before, *::after {
   box-sizing: border-box;
   margin: 0;
@@ -5,8 +46,8 @@
 }
 
 body {
-  background: #0f0f17;
-  color: #e0e0e0;
+  background: var(--bg);
+  color: var(--text);
   font-family: system-ui, -apple-system, sans-serif;
   display: flex;
   align-items: center;
@@ -26,7 +67,7 @@ body {
   font-size: 32px;
   font-weight: 800;
   letter-spacing: 4px;
-  color: #7aa2f7;
+  color: var(--accent);
 }
 
 .play-area {
@@ -41,10 +82,10 @@ body {
 
 #board {
   display: block;
-  border: 1px solid #2a2a3a;
-  background: #1a1a25;
+  border: 1px solid var(--border);
+  background: var(--board-bg);
   border-radius: 4px;
-  box-shadow: 0 0 40px rgba(0, 0, 0, 0.6);
+  box-shadow: 0 0 40px var(--shadow);
 }
 
 /* ---- Panel lateral ---- */
@@ -65,7 +106,7 @@ body {
 .label {
   font-size: 10px;
   letter-spacing: 2px;
-  color: #555570;
+  color: var(--label);
   font-weight: 600;
 }
 
@@ -73,14 +114,14 @@ body {
   font-family: 'Courier New', Courier, monospace;
   font-size: 26px;
   font-weight: 700;
-  color: #7aa2f7;
+  color: var(--accent);
   letter-spacing: 1px;
 }
 
 #next-canvas {
   display: block;
-  border: 1px solid #2a2a3a;
-  background: #1a1a25;
+  border: 1px solid var(--border);
+  background: var(--board-bg);
   border-radius: 4px;
 }
 
@@ -94,20 +135,20 @@ body {
 
 .controls li {
   font-size: 12px;
-  color: #888;
+  color: var(--muted);
   display: flex;
   align-items: center;
   gap: 4px;
 }
 
 kbd {
-  background: #22223a;
-  border: 1px solid #3a3a5a;
+  background: var(--kbd-bg);
+  border: 1px solid var(--kbd-border);
   border-radius: 3px;
   padding: 1px 6px;
   font-size: 11px;
   font-family: inherit;
-  color: #aaa;
+  color: var(--kbd-text);
 }
 
 /* ---- Overlay ---- */
@@ -117,7 +158,7 @@ kbd {
   display: flex;
   align-items: center;
   justify-content: center;
-  background: rgba(10, 10, 20, 0.75);
+  background: var(--overlay-bg);
   backdrop-filter: blur(4px);
   border-radius: 4px;
 }
@@ -138,19 +179,19 @@ kbd {
   font-size: 28px;
   font-weight: 800;
   letter-spacing: 4px;
-  color: #e57373;
+  color: var(--danger);
 }
 
 #overlay-score {
   font-family: 'Courier New', Courier, monospace;
   font-size: 14px;
-  color: #7aa2f7;
+  color: var(--accent);
 }
 
 #restart-btn {
   margin-top: 8px;
-  background: #7aa2f7;
-  color: #0f0f17;
+  background: var(--accent);
+  color: var(--accent-contrast);
   border: none;
   border-radius: 6px;
   padding: 10px 28px;
@@ -162,5 +203,33 @@ kbd {
 }
 
 #restart-btn:hover {
-  background: #9bbdff;
+  background: var(--accent-hover);
+}
+
+/* ---- Theme toggle ---- */
+.theme-toggle {
+  position: fixed;
+  top: 16px;
+  right: 16px;
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  background: var(--kbd-bg);
+  color: var(--text);
+  border: 1px solid var(--border);
+  border-radius: 999px;
+  padding: 6px 14px;
+  font-size: 12px;
+  font-weight: 600;
+  cursor: pointer;
+  transition: background 0.15s, border-color 0.15s;
+}
+
+.theme-toggle:hover {
+  border-color: var(--accent);
+}
+
+.theme-toggle:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
 }