More customizable css

(cherry picked from commit 8aa0899facd01f689692fccdb338673f5e9a9fbd)

_includes/css/custom.scss

+@import "./custom/custom";

_includes/css/just-the-docs.scss

 @import "./support/support";
 @import "./color_schemes/{{ include.color_scheme }}";
 @import "./modules";
-@import "./custom/custom";
+{% include css/custom.scss %}