Button and color enhancements (#24989) (#25176)

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">

templates/devtest/list.tmpl

@@ -1,3 +1,10 @@
+<style>
+	@media (prefers-color-scheme: dark) {
+		:root {
+			color-scheme: dark;
+		}
+	}
+</style>
 <ul>
 	{{range .SubNames}}
 	<li><a href="{{AppSubUrl}}/devtest/{{.}}">{{.}}</a></li>