Move all remaining colors into CSS variables (#21903)

This should eliminate all non-variable color usage in the styles, making
gitea fully themeable via CSS variables. Also, it adds a linter to
enforce variables for colors.

web_src/less/_dashboard.less

@@ -183,7 +183,7 @@
     }
 
     #privateFilterCheckbox .svg {
-      color: #888888;
+      color: var(--color-grey);
       margin-right: .25rem;
     }