Rework button coloring, add focus and active colors (#24507)

Rework button coloring, add focus and active colors (#24507)

We were missing overrides for `:focus` and `:active` styles which I've
added here along with two new color variants `dark-1` and `dark-2` for
them. Fomantic UI has 4 different colors but I think 3 are sufficient. I
also changed it on arc-green so button goes darker when pressed.

<img width="129" alt="Screenshot 2023-05-04 at 01 21 43"
src="https://user-images.githubusercontent.com/115237/236072060-7389276a-275b-4d3e-aa52-20b37c6e6d92.png">
<img width="130" alt="Screenshot 2023-05-04 at 01 17 59"
src="https://user-images.githubusercontent.com/115237/236071818-0e46414a-33db-4bb2-a3bd-35b514a8a2d0.png">
<img width="129" alt="Screenshot 2023-05-04 at 01 18 07"
src="https://user-images.githubusercontent.com/115237/236071819-562b1e38-541f-432b-b3b6-48e6d7594d00.png">
<img width="131" alt="Screenshot 2023-05-04 at 01 18 13"
src="https://user-images.githubusercontent.com/115237/236071820-89b7dba9-ce6c-48e5-a075-9053063e6ad3.png">
<img width="133" alt="Screenshot 2023-05-04 at 01 18 30"
src="https://user-images.githubusercontent.com/115237/236071823-b6fe2df4-b3f0-4dc8-97a8-f90ba6d19bec.png">
<img width="133" alt="Screenshot 2023-05-04 at 01 18 40"
src="https://user-images.githubusercontent.com/115237/236071824-b02ce61a-2367-4c29-8a25-45f231f5e5ee.png">

One misc change includes some fixes to editor and slightly darker
selection.

<img width="1245" alt="Screenshot 2023-05-28 at 19 16 19"
src="https://github.com/go-gitea/gitea/assets/115237/1ea4a4b6-26ba-45af-9cbc-5b8c476c2338">