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/vendors/_all.scss

@@ -16,3 +16,5 @@
  * You should have received a copy of the GNU Affero General Public License
  */
 
+@import "bulma-variables-overload";
+@import "../../../lib/bulma-scss/bulma";