And set it via 'gsettings set org.gnome.desktop.interface gtk-key-theme "supertheme"'
Theme is working, if I use Alt instead of Super - everything work fine in all applications. If I use Super - my keybindings work only in FF, which, I guess, read them from gtk and apply by itself. Why Super don't work and how can I fix it? Shortcuts with Super works perfectly when added via interface or gsettings system-wide (e.g. change desktops) and in GTK apps (e.g. in gnome-terminal). So system read Super and applications can handle it correctly. I've tried Meta, Mod4 and Hyper instead of Super - nothing works.
I think, it's a bug in GTK3, but may be I'm wrong. What can I do to fix it?