feat(swaync): Add addCss option for swaync

[Nerglej]

See #1953

I'm very sorry for the scarce documentation for this PR. I'm not that used to submitting PR's like this. I hope someone can correct my errors.

---

• 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]

I'm very sorry for the scarce documentation for this PR. I'm not that used to submitting PR's like this. I hope someone can correct my errors.

LGTM, except for one nitpick. For reference, this PR is essentially the same as commit e81bd0c ("waybar: add addCss option (#804)").

I've already made the code (that I would use), but I couldn't get the development project to run correctly with my config, caused by gnome-shell inputs, and then I ran out of time. Therefore I figured this issue would be appropriate instead of a pull request with untested code.

-- #1953

AFAICT, this PR should work. Do you still want to test this PR?

The request-reviewers action fails for the same reason as mentioned in #1931 (comment). Pinging @TheMaxMur, in case you want to take a look at this.

[trueNAHO]

Nitpick:

Suggested change

	extraOptions = {
	addCss = lib.mkOption {
	type = lib.types.bool;
	default = true;
	description = "adds fully functional css (otherwise just adds colors and fonts)";
	};
	};
	extraOptions.addCss = lib.mkOption {
	type = lib.types.bool;
	default = true;
	description = "adds fully functional css (otherwise just adds colors and fonts)";
	};