zen-browser: fix selection colors after Firefox 154

[saadndm]

The text selection theming introduced in #2184 and the urlbar text selection highlight no longer works with Firefox 154 after mozilla-firefox/firefox@fe629a3. It now ignores ::selection from user stylesheets, and instead falls back to ui.highlight and ui.highlighttext preferences.

This PR sets those ui.highlight and ui.highlighttext preferences to base0D and base00.

• These match the colors previously used for URL bar text selection
• zen-browser: add text selection theming #2184 used base02 and base05 for webpage selection. After Firefox 154 we can no longer keep webpage selection colors separate from the browser UI selection, and (personally) this new accent pair has better contrast in most cases.
• The existing ::selection rule is still needed because it overrides selection color defined by websites, but changed it to use base0D and base00 for consistency.
• Removed the URL bar selection rule which is not needed anymore

Existing style before 154	Existing style after 154	This PR after 154

Note: I did not update the testbed's zen-browser flake because it requires ffmpeg_9 which is not in the pinned nixpkgs version.

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• I have disclosed the scope of involved LLM tools, if any
• Commit messages adhere to Stylix commit conventions
• Theming changes adhere to the Stylix style guide
• Changes have been tested locally
• Changes have been tested in testbeds
• Each commit in this PR is suitable for backport

[MrSom3body]

LGTM, thanks!

[trueNAHO]

The change itself looks good, except for minor things. Thanks for the detailed explanation.

I did not update the testbed's zen-browser flake because it requires ffmpeg_9 which is not in the pinned nixpkgs version.

Unless this causes non-trivial and unrelated issues, consider updating both zen-browser and nixpkgs inputs.

[trueNAHO]

Could you use the withHashtag variant here? Although /modules/zen-browser/userChrome.nix does not use it either, it might be good to keep at least this file pristine. Polishing /modules/zen-browser/userChrome.nix is out of the scope of this PR.