N°7793 Add breaking changes introduced by adding common SCSS variables between backoffice and end-user portal

[steffunky]

Base information

Question	Answer
Related to a SourceForge thead / Another PR / Combodo ticket?	N°7793
Type of change?	Enhancement

Objective

This PR add the breaking changes that were explained in #674 but not added as it was on 3.2.x LTS branch

Proposed solution

All 'helpers' $ibo- variables now take their default values from a $common- ancestor.
That may lead to breaking changes in themes (see bf4de40 as an example).

From #674 description

Up until now $ibo- variables that inherited another $ibo- variable value could be indirectly overloaded with a theme:

But with how we envisioned the addition of $common variables, it would break the existing themes by breaking the link between $ibo- variables

In iTop 3.2.1 we'll keep this behavior and add a breaking change in iTop 3.3.0 that will require themes to either overload both $ibo- variables, or overload the $common- variable

Checklist before requesting a review

• I have performed a self-review of my code
• I have tested all changes I made on an iTop instance
• Is the PR clear and detailed enough so anyone can understand digging in the code?