The styles for these buttons when removing the mixing clash between
https://github.com/xwiki/xwiki-platform/blob/de55262bf6fcdd1ab49cae3e0c6e791d01f0412b/xwiki-platform-core/xwiki-platform-bootstrap/src/main/less/buttons.less#L69
and
https://github.com/xwiki/xwiki-platform/blob/0fbc4ccbae83e56f8fbff12964f53a97b6c0ecda/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/less/buttons.less#L23-L24
Right now in XWiki those default buttons have a white background, however for consistency we'd like something light grey.
