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/_install.less

@@ -59,7 +59,7 @@
     .reinstall-message {
       width: 70%;
       margin: 20px auto;
-      color: red;
+      color: var(--color-red);
       text-align: left;
       font-weight: bold;
     }