This widget, like others implemented with OOUI, is painful to style for wiki admins who maintain a custom theme but only have interface admin access. As far as I understand, the "correct" method for styling OOUI is described in OOUI/Themes and would require access to the server.
Topic on Talk:Edit Review Improvements/New filters for edit review
I think you would have better chances to have a reply if posting on the OOUI talk page.
Thanks for pointing me to the right direction. I'll give it a go.