feat: spa light/dark mode

[lelenei]

Description

Closes # mf-4786

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.