docs: add variables-dark import in home page

Author: tiendq

site/layouts/partials/home/customize.html

@@ -41,6 +41,7 @@ <h3>Include what you need</h3>
 
 // Required Bootstrap imports
 @import "../node_modules/bootstrap/scss/variables";
+@import "../node_modules/bootstrap/scss/variables-dark";
 @import "../node_modules/bootstrap/scss/maps";
 @import "../node_modules/bootstrap/scss/mixins";
 @import "../node_modules/bootstrap/scss/root";