I prefer the breeze-dark KDE theme. Unfortunately, some gnome apps (such as Firefox) are problematic. In particular, in text input fields in Firefox, one ends up with white text on a white background or dark text on a dark background.
Settings -> colors -> options -> apply colors to non-Qt applications enabled Settings -> Application Style -> Gnome Application Style (GTK): GTK3 Theme: Adwaita-dark Icon theme: Adwaita
I am not aware of any Adwaita-dark icon theme after extensive searching.
To enable breeze-dark for KDE I have:
Settings->Application Style-> Widget Style: Breeze Settings->Workspace Theme -> Desktop Theme: Breeze Dark
It should not be necessary, but I have also installed https://addons.mozilla.org/en-Us/firefox/addon/breeze-dark/.
[Settings] gtk-application-prefer-dark-theme=true gtk-button-images=1 gtk-cursor-theme-name=ComixCursors-Opaque-Orange gtk-fallback-icon-theme=Numix-Circle gtk-font-name=Liberation Sans Regular 11 gtk-icon-theme-name=Adwaita gtk-menu-images=1 gtk-primary-button-warps-slider=1 gtk-theme-name=Adwaita-dark gtk-toolbar-style=GTK_TOOLBAR_ICONS
After all of this, Firefox text input fields still have either white text on a white background or dark text on a dark background, making them impossible to read. (Temporary workaround is to highlight the text in a field so I can see what was entered, but that is very clumsy.)
The following question claim this closely related issue was a bug that was fixed: KDE - Problem with dark themes
However, I a running Arch Linux with the latest KDE Plasma5 and what appears to be the same issue still exists. However, I notice it mainly in text input fields, not necessarily drop down combo boxes.