Move from custom.css to custom.scss to be able to use variables

assets/book.scss

@@ -217,4 +217,7 @@ aside nav,
   .book-toc nav {
     padding: $padding-16 * 2 $padding-16;
   }
-}
+}
+
+// Add custom defined styles
+@import 'custom'

assets/custom.scss

@@ -0,0 +1,2 @@
+/* To be overridden by theme user */
+// @import "variables";