Theoretically you could write-protect it with the immutable flag
sudo chattr +i /usr/share/sddm/themes/Sweet/theme.conf.user
Just an idea... There might be better solutions.
For example you could name your background
Dragon_wp.png, but if we change the standard background (and name) you're lost. So probably the flag is a better option.
This is the file which is overwriting your change at every update of the dr460nized settings