After checking the code, it seems like it's a bug. There's some LESS variables in the CSS. My guess is that both the new LESS variables and LESS->CSS migration were done around the same time and it wasn't coordinated so we didn't catch the error.
https://github.com/xwiki/xwiki-platform/pull/3881
It's technically not a regression since those pagination elements were completely different before
Started a PR at https://github.com/xwiki/xwiki-platform/pull/4910