Use CSS Vars for primary color (#13361)

Use CSS Vars for primary color (#13361)

* Use CSS Vars for primary color

- Create 15 color shades derived from primary color
- Change blue-ish colors to use that primary color
- Move styles for scrollbars, selection from arc-green to base
- Remove obsolete arc-green styles that now use those variables

* simplify webkit scrollbar style

Co-authored-by: techknowlogick <techknowlogick@gitea.io>