I have problems with some customization of my OS (Ubuntu 16.04.3 LTS).
I changed wallpapers and monitor position (reversed them) using GUI tools (Appearance & Displays in System settings) and everything turned out without any problems or errors.
But every day, when I power on or reboot PC, on login screen I see the classic theme, and mouse works as if the right monitor is the left and so on.
After login all works (and looks) right.
How can I fix it?