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/_base.scss

@@ -0,0 +1,9 @@
+/*!
+ * @copyright   Copyright (C) 2010-2021 Combodo SARL
+ * @license     http://opensource.org/licenses/AGPL-3.0
+ */
+
+// These are the base variables used throughout the backoffice, if you don't know what to use, these are probably good :)
+
+$ibo-base-variable--text-color: $ibo-color-grey-900 !default;
+$ibo-base-variable--border-radius: $ibo-border-radius-300 !default;;