Add dark mode to API Docs (#24971)
Add a dark mode to Swagger UI via CSS `invert`. No toggle or anything,
but I think it's better than nothing. Users can toggle via their OS.
Also includes a few misc CSS cleanups on the page.
<img width="1264" alt="Screenshot 2023-05-28 at 20 25 06"
src="https://github.com/go-gitea/gitea/assets/115237/de761b85-ca0c-4220-bee4-73798a4360a0">
<img width="1260" alt="Screenshot 2023-05-28 at 20 02 54"
src="https://github.com/go-gitea/gitea/assets/115237/29188ed2-c167-47f5-bf28-46193d1da22c">
faf8557b4c