Backport #24989. Clean cherry-pick aside from one small conflict with divider. - Various corrections to button styles, especially secondary - Remove focus highlight, it's annoying when it stays on button after press - Clearly define ghost and link buttons with demos in devtest - Remove black, grey and tertiary buttons, they should not be used - Make `arc-green` slightly darker <img width="1226" alt="image" src="https://github.com/go-gitea/gitea/assets/115237/8d89786a-01ab-40f8-ae5a-e17f40e35084"> <img width="1249" alt="image" src="https://github.com/go-gitea/gitea/assets/115237/83651e6d-3c27-46ff-b8bd-ff344d70e949">
Reorganize various CSS files for clarity, group together by subdirectory in `index.css`. This reorders some of the rules, but I don't think it should introduce any issues because of that.
