I'm trying to create an Avendor subtheme to customize everything to my liking.

However, just making an info file (copied from basetheme so regions are correct) everything looks alright except MegaMenu. It seems like it's functional but it's missing some styling (e.g. all base menu items are just text right next to eachother, missing padding/margin, ...). 

I can't find any specifics on subtheming Avendor. What am I doing wrong?