Customize: Improve display of theme tag filters on mobile

[celloexpressions]

Split out from #42212.

The filter bar disappears on screens shorter than a magic number (or narrower than a different, also magic number), unless you scroll all the way up to the top (quite difficult with one-column infinite scroll of themes). I don't happen to use this feature on any devices that would be affected by this problem, but the experience is quite subpar for those users currently. I'm guessing there might be some issues with CSS caching in the above screenshots, or, those that worked on the previous changes in this ticket may have a better idea of why certain browsers don't like something here. Probably worth exploring before 4.9 release if anyone is able to do that, though.