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";

css/backoffice/vendors/_bulma-variables-overload.scss

@@ -0,0 +1,19 @@
+/*!
+ * Copyright (C) 2013-2020 Combodo SARL
+ *
+ * This file is part of iTop.
+ *
+ * iTop is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * iTop is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ */
+
+$family-sans-serif: "Monorale";