Convert LESS SSX to CSS SSX

This ticket might need to be split down further depending on implementation complexity.
At the creation of this ticket, I assume it can be closed given the specific SSX that have not been converted have been reported in their own improvement ticket.

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Thanks to CSS variables, we should be able to move most of the SSXs in XS to native CSS. Doing this will improve performance because the use of LESS comes with a costly compilation phase.

This is a part of XWIKI-22668.

For reference, in 17.10.X, I could get a search that found 27 SSXs in the whole `xwiki-platform` codebase:

https://github.com/search?q=repo%3Axwiki%2Fxwiki-platform%20%22%3CcontentType%3ELESS%3C%2FcontentType%3E%22&type=code