vscode: improve theme

[IanHollow]

WIP Improving the vscode theme

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• [x] I certify that I have the right to submit this contribution under the MIT license
• [ ] 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 to the current stable branch

[trueNAHO]

Btw, you can also locally test your Stylix branch without remotely pushing with something like:

inputs.stylix.url = "git+file:///home/user/path/to/stylix";

or

inputs.stylix.url = "path:///home/user/path/to/stylix";

The documentation also mentions this, although the path: protocol is not mentioned because it is a relatively new nix feature:

https://github.com/nix-community/stylix/blob/f19ac46f6aa26188b2020ed40066a5b832be9c53/doc/src/modules.md?plain=1#L3-L23