Gtk common themes doesn't correctly apply breeze dark under wayland in firefox on kubuntu

I’ve noticed something interesting about running Firefox on Kubuntu: whenever Firefox is run under Wayland, it is able to access the Breeze Light GTK theme just fine from gtk-common-themes, but not Breeze Dark. Instead it falls back to Adwaita-Dark.

Interestingly, it is able to access Breeze Dark whenever Firefox is instead run under X11 or XWayland. I wonder why this is…?

I did also report this to Mozilla’s bug tracker, but was hoping it would be useful if the snap people could also have a look at this.