Expose uPortal CSS variable

[jgribonvald]

Is your feature request related to a problem? Please describe.
For web-components it would be great to adopt directly uPortal css variable like those done in the past for portlets.
It would avoid to add to the uPortal skin several variables defined on each web-components.

Describe the solution you'd like
Expose/define uPortal CSS variable that could be used into web-components like we do for

:root {
   --portlet-header-text-color: #000000
   --portlet-content-bg-color: #d0d0d0
...
}