Make LESS customizations impact CSS properties

As of XWIKI-22667, we do have a set of CSS properies. However, any customization of the LESS equivalents does not work on their equivalent CSS properties.

It is expected that CSS properties that have an equivalent in LESS always have the same value of this equivalent by default. It would make the migrations way safer.

Customizations that are not currently reflected in CSS variables are contained in the FlamingoColorTheme Advanced `@lessCode` and SSXs for example.

I think it's fair to only use the customizations at the :root level. For any customization of the value of a variable which is not at the root level, the migration will need to be handled by hand.