N°2847 - Improve SCSS files structure

- Refactor some mixins into helpers
- CSS3 variables set via SCSS variables are now using "interpolation" (#{$my-var} instead of just $my-var) to be compatible with more SCSS compiler
- Overloading some Bulma variables such as font-family

css/backoffice/utils/_all.scss

@@ -17,5 +17,7 @@
  */
 
 @import "variables/color";
+@import "variables/border-radius";
 @import "variables/typography";
-@import "mixins/typography";
+@import "mixins/all";
+@import "helpers/all";