SCSS: Fix rules from 108bc2fa

- Don't nest selectors when there is no need to
- Use SCSS variables instead of CSS3 variables in SCSS files, otherwise we can't use SCSS functions. CSS3 variables should be kept for the front-end manipulations

css/backoffice/utils/variables/_all.scss

@@ -9,3 +9,5 @@
 @import "elevation";
 @import "path";
 @import "typography";
+// Important: Keep this partial last as it includes variables from the previous partials
+@import "base";