refactor: pick theme refactor from GH-6009

[Jack-Works]

Description

Closes # (NO_ISSUE)

Type of change

• Documentation
• Code refactoring (Restructuring existing code w/o changing its observable behavior)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (a fix or feature that would make something no longer possible to do/require old user must upgrade their Mask Network to this new version)

Previews

Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
  • I have removed all in development console.logs
  • I have removed all commented code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have read Internationalization Guide and moved text fields to the i18n JSON file.

If this PR depends on external APIs:

• I have configured those APIs with CORS headers to let extension requests get passed.
  • chrome extension: chrome-extension://[id]
  • firefox extension: moz-extension://[id]
• I have delegated all web requests to the background service via the internal RPC bridge.

[Jack-Works]

This is picked from #6009, the refactor to src/utils/theme*. I split them into multiple files, with no runtime behavior changes. I'll merge it once CI passed.

[github-actions]

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