[dajiaohuang]

Summary

The path-based PlantumlContext.Export overload passed a null theme to DiagramBuilder.Build, unlike the no-path overload, which supplies DefaultTheme. C4SCLI uses the path-based overload without a theme, so its generated PlantUML lacked the library's default style directives. Apply the same default theme in the path overload.

Validation

• A local source-only .NET 10 harness used a synthetic C4 diagram and generated PUML files only (no image renderer, Java process, or external service). Before the fix, the no-path output contained default style directives while the path output did not. After the fix, both outputs are identical and contain the default style.
• git diff --check passed.
• No repository test project was found. The full solution/CLI build and net8/net9 target checks were not run; an offline multi-target harness restore could not resolve those reference packs. No real user architecture files were accessed.