test: add unit tests for themes utility functions (#2718)

[desireddymohithreddy0925]

Description

Resolves #2718

This PR adds comprehensive unit testing for the pure utility functions exported from src/lib/themes.ts using Vitest. It ensures predictable theme resolution and safeguards the core behavior of the theme selection logic.

Changes Made

• Created test/themes.test.ts:
  • isThemeId: Validated parsing behavior for all known theme IDs and appropriately handled null, undefined, empty, and mis-cased strings.
  • getThemeDefinition: Confirmed it consistently retrieves the correct ThemeDefinition and provides a safe fallback (classic-dark) for unknown identifiers.
  • isDarkTheme: Verified expected boolean resolution for the app's three dark modes (classic-dark, nordic-frost, cyberpunk-matrix) and the single light mode (modern-light-blue).
  • nextThemeId: Assured deterministic cycling through the THEME_OPTIONS array, including sequential iteration, list wrap-around, and handling of unknown starting inputs.
  • Constants Validation: Assured THEME_OPTIONS maintains exactly 4 valid entries and that DEFAULT_THEME remains pinned to classic-dark.

Acceptance Criteria

• Unit tests for isThemeId, getThemeDefinition, isDarkTheme, and nextThemeId cover all theme variants alongside fallback scenarios.
• Core module constants are asserted correctly, providing documentation-through-tests.
• The module now achieves 100% test coverage.
• All 12 tests run successfully and deterministically with vitest.

[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

[Legit-Ox]

Logic is correct — all assertions match the source. Two things to fix before merging.

[Legit-Ox]

Wrong import path. Other test files in this project use the @/ alias (e.g. @/lib/streak, @/lib/streak-utils). This should be:

import { ... } from '@/lib/themes';

'../src/lib/themes' is a relative path that breaks if the test file moves and diverges from the established convention.

[Legit-Ox]

Fragile length assertion. expect(THEME_OPTIONS).toHaveLength(4) will fail the moment a fifth theme is added — even if the new theme is perfectly valid and all other tests pass. This is testing an incidental implementation detail, not a behavioral contract.

Better approach — assert the known IDs are present rather than the exact count:

it('contains all expected theme IDs', () => {
  const ids = THEME_OPTIONS.map((t) => t.id);
  expect(ids).toContain('classic-dark');
  expect(ids).toContain('modern-light-blue');
  expect(ids).toContain('nordic-frost');
  expect(ids).toContain('cyberpunk-matrix');
});

[Priyanshu-byte-coder]

Closing as duplicate. #2774 was selected as the themes test winner for its maintainability (uses loops over THEME_OPTIONS). Thanks for contributing!