There’s no need to have the button zone visible if the item in the button itself has enough contrast.
A button which has a distinguishing indicator such as position, text style, or context does not need a contrasting visual indicator to show that it is a button, although some users are likely to identify a button with an outline that meets contrast requirements more easily.
From Understanding Success Criterion 1.4.11: Non-text Contrast | WAI | W3C
So as long as the icon inside respects contrast, we’re good 
Considering the hover state, for now on my end I consider it as a best effort. There’s a lot of moving parts in XWiki and sometimes it’s not possible to have a clean style for hover that would satisfy all contrast needs. So 
for this solution from me.
Thanks,
Lucas C.