Improve basic button and label styles (#14119)

* Improve disabled styles for repo buttons

- Simplify disabled styling of label by matching for the disabled
  attribute.
- Raise fomantic disabled opacity from .45 to .55 to for more contrast.
- Use CSS vars for basic button styles.

* restore clickability on label

* color tweaks and remove arc-green style

* slightly reduce button size

* consolidate vars

* also cover active class

* slightly more distinct active class

* remove useless rule

web_src/fomantic/_site/globals/site.variables

@@ -4,3 +4,4 @@
 @pageFont: var(--fonts-regular);
 @bold: 500;
 @useCustomScrollbars: false;
+@disabledOpacity: var(--opacity-disabled);