XWiki console prints warning when user is trying to apply style with CSS 3 HEX background-color rule

Manuel Leduc sorry I didn't see your comments. I confirm that the "Invalid color" was indeed the error I was referring to.

Also CSS Parser does not seems to be under active development: latest version is almost two year old and https://sourceforge.net/p/cssparser/bugs/88/ didn't received any reply in more than 4 years.
So I don't really have hopes to get a fix in CSS Parser.