Add a new tab "Customized Variables" on the color editor

I propose to improve the color theme editor to make it more powerful and flexible, for this i propose :

• Add a new tab "Customized Variables" on the editor, this tab where allow us to add more LESS CSS variables and list them on a list, we can use this variable for adding more LESS css rules on the advanced tab and target any area on the plateforme.

http://design.xwiki.org/xwiki/bin/view/Proposal/Improvetheskineditor