test : added unit tests for themes.ts

[tmdeveloper007]

Closes #2894.

Summary of What Has Been Done: Created test/themes.test.ts with vitest covering all exported functions from src/lib/themes.ts: isThemeId, getThemeDefinition, isDarkTheme, nextThemeId, plus THEME_OPTIONS and DEFAULT_THEME constants.

Changes Made: 47 lines of test coverage across 16 test cases including theme options, dark mode detection, ID validation, and theme cycling logic.

Impact it Made: Adds test coverage for a previously untested utility module. Ensures theme selection and dark mode logic behave correctly.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — 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