Fixed reference to old variable name (#103)

Using RTL would fail due to _colors no longer being available. Changed to import _variables instead.

assets/scss/coder-rtl.scss

@@ -1,4 +1,4 @@
-@import "_colors";
+@import "_variables";
 @import "_base_rtl";
 @import "_content_rtl";
 @import "_navigation_rtl";