added p1 changes to public/src/admin/appearance/themes.js

[minghanm]

This pull request refactors the themes.js file located in public/src/admin/appearance/ to address the issue of deep function nesting (more than 4 levels deep).

Testing was done locally.

Resolves https://github.com/CMU-313/NodeBB/issues/496

[coveralls]

Pull Request Test Coverage Report for Build 10975433920

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 82.685%

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Totals
Change from base Build 10975095078:	0.005%
Covered Lines:	22331
Relevant Lines:	25587

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💛 - Coveralls