test : added unit tests for themes helper functions

[tmdeveloper007]

Closes #2612.

Summary of What Has Been Done: Added unit tests for the pure utility functions exported from src/lib/themes.ts. The test file covers isThemeId, getThemeDefinition, isDarkTheme, nextThemeId, and the module constants.

Changes Made:

• Created test/themes.test.ts with 16 test cases
• isThemeId: tested all 4 valid IDs, null/undefined, and invalid strings
• getThemeDefinition: tested each theme returns correct definition, fallback to classic-dark for unknown IDs
• isDarkTheme: verified dark-mode themes return true, light-mode returns false
• nextThemeId: tested forward cycling through all themes, wrap-around from last to first, graceful handling of unknown IDs
• Added tests for THEME_OPTIONS length and DEFAULT_THEME constant

Impact it Made: Ensures theme utilities are correctly validated and documented. Guards against regressions when theme options are added or modified.

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

CI is failing on this PR. Please fix the TypeScript/test errors before we can merge. Run npm run typecheck and npm test locally to reproduce.

[tmdeveloper007]

This PR has been closed as it has been stale for more than 2 days without recent activity or merging. If this work is still relevant, please reopen a fresh PR.