from the command line. To add a new background for a particular theme copy that image into the theme folder as root. Then in your theme edit the file theme.conf as root and change the line background= with the path to your image. To edit theme.conf as root run
from the command line. To add a new background for a particular theme copy that image into the /usr/share/sddm/theme/ folder as root. Then in your theme edit the file theme.conf as root and change the line background= with the path to your image. To edit theme.conf as root run