I was trying to add a new monitor to system.
There was no additional monitor in
Monitors settings section.
nvidia-settings application found other monitor, but after it's activation nothing happened. After that, I turned on
xinerama option and after X restart second monitor got connected.
But now most of "background colors" in applications turned into back, UI theme turned into default Ambiance and application context menus looks like old-styled GTK apps. Also transparency and hiding of "File, Edit" menus doesn't work at all.
Trying to set another Theme in "View" section makes no sence. I had adwaita theme before that.
Moreover, "Xlib: extension "RANDR" missing on display ":0"." error appears from time to time.