Fixes Bug 1046746 - Adding top margin to captions below thumbnails.

Mixing `em`s and `rem`s can be dangerous unless you’re specifically targeting something you want to behave like a component. (`rem`s in <webmaker.org> = 10px)

I have been *trying* to remove most `px` & `em` measurements (except where contextually relevant) from wm.o as we make changes, so something to think about.