vscode: improve theme

[IanHollow]

WIP Improving the vscode theme

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• 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:

stylix/doc/src/modules.md

Lines 3 to 23 in f19ac46

	## Development setup
	Currently the easiest way to test Stylix is to use the new code in your actual
	configuration.
	You might find it useful to override Stylix' input flake reference on your
	flake, from `github:nix-community/stylix` to
	`git+file:/home/user/path/to/stylix`, so that you don't need to push changes to
	GitHub during testing.
	To do that, instead of editing your `flake.nix`, you can leverage `nix`'
	`--override-input` parameter (which can also be supplied through their
	frontends: `nixos-rebuild`, `nix-on-droid` and even `nh`). It allows you to
	deploy your changes in one fell swoop, without having to update the lock file of
	your flake every time you make an edit.
	Just append `--override-input stylix git+file:/home/user/path/to/stylix` to your
	standard `nix` (or `nix` frontend) incantation.
	Nix only reads files which are tracked by Git, so you also need to `git add
	«file»` after creating a new file.