test : added unit tests for themes.ts

[tmdeveloper007]

Closes #2758.

Summary of What Has Been Done: Added a vitest test file test/themes.test.ts covering all exported pure functions from src/lib/themes.ts.

Changes Made:

• Created test/themes.test.ts with 23 tests across 4 describe blocks.
• isThemeId: tested all 4 valid theme IDs (true) and invalid inputs (false) including empty string, null, undefined, and partial matches.
• getThemeDefinition: tested each valid theme ID returns correct ThemeDefinition with id, name, mode. Tested invalid ID falls back to classic-dark.
• isDarkTheme: tested classic-dark, nordic-frost, cyberpunk-matrix return true; modern-light-blue returns false.
• nextThemeId: tested correct cycle order, wrap-around from cyberpunk-matrix to classic-dark, and invalid ID fallback.

Impact it Made: Theme selection is user-facing in settings and on the public profile. These functions are the single source of truth for theme metadata; testing ensures theme IDs are validated correctly and the cycle navigation works without regression.

Hello @Priyanshu-byte-coder, please assign this issue to me (@tmdeveloper007) so I can open a focused PR.

[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]

Closing as duplicate. #2774 was selected as the themes test winner — it uses loops over THEME_OPTIONS making tests maintainable if themes are added, rather than hardcoding every theme ID. Thanks for contributing!