fix: preserve mode-dependent overrides in dark mode rules

[SpyZzey]

Fixes AWSUI-61881: The dark mode gets overridden when light and dark mode uses the same value in themes due to some optimizations.

Issue #, if available: AWSUI-61881

How to test it locally:

• Create a simple react project that consumes cloudscape-design from npm
• Create a custom theme where a token has the same light as dark value (see appendix below)
• Checkout theming-core and build the theming-core project, copy the lib/browser/browser and lib/browser/shared folder and replace the corresponding ones in the node_modules/@cloudscape-design/theming-runtime package or use npm link.
• Run npm run dev and check how it looks like (and compare it with the current live theming-runtime from @cloudscape-design/theming-runtime)

Appendix:

applyMode(Mode.Dark);
applyTheme({
  theme: {
    tokens: {
      colorBorderButtonNormalDefault: {
        light: "pink",
        dark: "pink",
      },
    },
  },
});

export default function App() {
  return (
    <div style={{ padding: "20px" }}>
      <Button>Test</Button>
    </div>
  );
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.59%. Comparing base (43861b7) to head (8d85739).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #164      +/-   ##
==========================================
+ Coverage   97.57%   97.59%   +0.01%     
==========================================
  Files          48       48              
  Lines        2641     2657      +16     
  Branches      549      558       +9     
==========================================
+ Hits         2577     2593      +16     
  Misses         64       64              

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[pan-kot]

Why is it necessary to preserve mode rules in this case?

E.g.:

body {
  color-badge-status-critical: red;
}
.dark-mode {
  color-badge-status-critical: red; // Is this needed?
}