fix(opencode): use themed TUI overlay dimmers

[ChaoGlenXu]

Issue for this PR

Closes #13363

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes TUI dialogs and overlay dimmers using a hardcoded translucent black backdrop, which looks wrong on light terminals when using the system theme.

The current code hardcodes black overlay colors for the dialog backdrop and the narrow-screen session sidebar overlay. This change replaces those hardcoded colors with a shared theme-aware overlay helper that derives the overlay color from the current TUI theme, while preserving the existing dimmer opacity.

That keeps dark terminals dark, but avoids black-looking overlays on light terminals.

How did you verify your code works?

I added a focused test for the overlay color helper and ran:

bun test test/cli/tui/overlay.test.ts

I also ran:

bun run typecheck

And I verified the branch push passed the repo pre-push hook, including:

bun turbo typecheck

Screenshots / recordings

Not included. This is a small TUI overlay color fix and I verified it through targeted tests and typecheck.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potentially related PR:

• PR fix(tui): use theme-aware color for dialog overlay on system theme #14686: "fix(tui): use theme-aware color for dialog overlay on system theme" - fix(tui): use theme-aware color for dialog overlay on system theme #14686

This PR appears to be directly related to the current PR #18750, as both address the same issue (theme-aware overlay colors for TUI dialogs). You may want to verify if #14686 was previously closed/abandoned, or if #18750 is a fresh attempt to fix the same problem. Additionally, PR #13239 ("fix(tui): fix system theme contrast with transparency") may be tangentially related to theme contrast issues.

[atharvau]

Code Review Summary

This PR fixes TUI overlay colors to be theme-aware instead of using hardcoded black backgrounds. It's a well-executed bug fix with good testing and clean implementation.

✅ Strengths

• Clear Problem/Solution: Addresses the specific issue of hardcoded black overlays on light terminals
• Clean Implementation: New overlayColor helper follows single responsibility principle
• Good Test Coverage: Comprehensive unit tests covering all fallback scenarios
• Backwards Compatible: Preserves existing opacity behavior while fixing the color issue
• Consistent Application: Fixes the issue in both dialog backdrops and session sidebar overlays

✅ Code Quality

• Type Safety: Proper TypeScript types with good interface definition
• Error Prevention: Handles transparent colors gracefully with fallback logic
• Readable: Clear function logic with good naming conventions
• Testable: Pure function that's easy to unit test

✅ Testing

• Comprehensive Coverage: Tests primary use case, fallbacks, and custom opacity
• Good Test Structure: Clear test descriptions and expected behavior
• Edge Cases: Handles transparent backgroundMenu and backgroundPanel scenarios

🔍 Minor Observations

1. Style Conformance: The code follows the established patterns well
2. Performance: Minimal overhead - just color calculation, no performance concerns
3. Maintainability: Helper function is easy to extend if more overlay types are needed

🎯 No Issues Found

• ✅ No security concerns
• ✅ No bugs detected
• ✅ No performance issues
• ✅ No style violations
• ✅ Good error handling
• ✅ Proper type safety

📊 Test Coverage: Excellent ✅

• Unit tests cover all code paths
• Edge cases properly tested
• Integration with existing theme system verified

🚀 Recommendations

This PR is ready to merge as-is. The implementation is clean, well-tested, and solves the stated problem effectively.

Overall Assessment: Approved ✅

This is a high-quality bug fix that improves the user experience without introducing any risks or technical debt.