test : added unit tests for themes utilities

[tmdeveloper007]

Closing in favour of fix PR #2705 — the themes test had a TypeScript error (passed ThemeDefinition object instead of ThemeId to getThemeDefinition). The corrected version is at #2705

[github-actions]

GSSoC Label Checklist 🏷️

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

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