chore: Split base theme into a separate file that can be more easily removed/replaced at build-time

[avinashbot]

Description

See Chorus/rzZV6TJY6Zav. What if, instead of creating a themed "copy" of the components libraries, you just create a stylesheet that contains all the design tokens that the components need with the values set to the themed values? And then, you can either override it (using cascade layers or putting it later in the DOM order) or just replace this file (using bunder overrides).

This is to support the replacement strategy. Nothing in theming core changes intentionally, so it's easy to revert, this is just to verify that moving the base styles doesn't break anything (technically, don't see why it should, but it's best to play it safe).

Related links, issue #, if available: n/a

How has this been tested?

Doing a dry-run (7976441604).

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.66%. Comparing base (3bbec78) to head (043e4dc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4855   +/-   ##
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  Coverage   97.66%   97.66%           
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  Files         958      958           
  Lines       31314    31314           
  Branches    11564    11565    +1     
=======================================
  Hits        30582    30582           
+ Misses        725      686   -39     
- Partials        7       46   +39     

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