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>

web_src/less/_review.less

@@ -145,11 +145,11 @@ a.blob-excerpt {
   justify-content: center;
   align-items: center;
   width: 100%;
-  background: #daecfe;
+  background: var(--color-primary-light-5);
 }
 
 a.blob-excerpt:hover {
-  background: #428bca;
+  background: var(--color-primary);
   color: #fff;
 }