Improve basic button and label styles (#14119)

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