N°7793 Add 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 feature aims to ease future portal style rework by adding common SCSS variables shared between the portal and the backoffice.
This will ensure that portal and backoffice maintain a similar style and ease overloading both frontends with a single customization.

Proposed solution

This approach adds a set of variables named $common- as opposed to itop backoffice $ibo- that will distribute default values for front ends.

$ibo- variables now consume $common- values that are then fed to the stylesheets.

To avoid breaking existing themes in a maintenance release, not all variables can inherit $common- values.
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?

Checklist of things to do before PR is ready to merge

[x] Ensure previous themes are not broken by this change
=> No differences in custom light/dark themes when comparing dashboards, lists, details
[x] Ensure all variables that can be made common in iTop 3.2.1 are changed in this PR

[Molkobain]

Nice job, maybe you could duplicate the README.md & _shame.scss (empty) into the common folder as well.