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?
This is because you are using user settings, instead of using them globally and they apply only after logging in with the specific user. Put the same settings in your display manager settings and everything will be fine.
Edit: I guess you are using lightdm as it's the default display manager for ubuntu, in that case you can put your
monitors.xml
in/var/lib/lightdm/.config/
so your monitors resolution and order will work at the login screen. As for the wallpaper on your login screen check this link.