So I've finally started to work on getting this dev site updated to the latest version of C5. Had a couple glitches, but finally made it to 5.6.x (whatever is latest), and the backend all looks right, although I'm seeing issues with the "Preview" option on Themes, front end even looks right, but there's one glaring problem -- the C5 toolbar at the top of the page is completely blank. It shows up, but there are no icons or buttons on it whatsoever (see attached screenshot). The site is using advanced permissions, but most/all of the options don't seem to be editable (ie the loading icon just spins and spins).

Any ideas of how to fix / where to start? The site is 200+ pages so I'm not inclined to just start over. Thanks!