After I installed the Radiance theme in Ubuntu 20.04 and set the system to use it, there are no horizontal menu separators in the menus. I noticed it in Firefox, because the separators I created between bookmarks didn't show up in the bookmarks menu (although they are correctly visible in Show All Bookmarks window), but the same is happening for other menus, eg. menus dropping from top GNOME panel (I use GNOME Flashback).
When switching to some of the default system themes, like Yaru or Adwaita, the separators are visible. What should I change in theme's CSS files to bring the separators back?