Using a dark theme in KDE with Gnome applications (e.g., Firefox)
Tested in Plasma 5.12 (Kubuntu 18.04) and 5.14 (18.10 upgraded to backports).
I can select "Breeze Dark" for the GTK themes.
I have breeze-gtk-theme
installed.
Also, look for other dark themes under "Settings -> Application Style -> Gnome Application Style (GTK)" - "Get new themes".
I have seen the Firefox problem when in the past I used dark Kvantum themes, but only in Firefox. Not happening now though.
My experience, GTK+ application theming doesn't work well if it's not gnome session.
You should try the lxappearance
application while running KDE to apply GTK+ theme or Icon theme. Tested on KDE5 and LXQt.
I think the Pop icon theme blend well with both dark and light themes.