There is a metrics.ini, but there is no BGAnimations or more folders, just a handful of .pngs and luas
Sounds like you've got noteskins in the theme folder. Put the noteskins (folders with only metrics.ini, pngs and luas) in NoteSkins/dance/ to be able to select them in-game to customise the look of the arrows themselves.
Check out this page for a big list of themes for Stepmania 5.