[Task] Add unit test cases for ThemePreview component

Task Description:
Currently, we run unit tests using jest and co-locate the test files with the source files as can be seen here.

However, the test cases are not yet comprehensive enough and work can be done to cover more grounds. Specifically for this issue, we hope to add test cases for the ThemePreview component.

Deliverable(s):
New unit test cases that cover the testing of the ThemePreview component.

Additional Context:

• The ThemePreview component can be found here
• An example for SearchBar can be found here which may serve as a good source for reference

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Start by inspecting the ThemePreview component under src/components/Themes and the existing SearchBar test at src/components/SearchBar/SearchBar.test.tsx. Run the repository's Jest tests to understand the setup, then add co-located ThemePreview test cases covering its documented behavior; done means the new tests pass and provide broader coverage.