Because the config file of sddm is under /usr/share it is not possible to install sddm themes and change background pictures as normal. @nullrequest has thought of making sddm themes a rpm package and use rpm-ostree to install it. see hear However, she only provides source code but not binary program which makes it really inconvenient to use. ...
the above fedoraproject linked hacky way works. i've done it myself. do note that your copied folder doesn't pick up any updates to the /usr folder though so you want to manually re-copy when sddm gets updated
@Kyle Gospo have reported the issue on the gnome gitlab. They may be potentially looking at a different approach for this kind of fractional scaling in future anyway so god knows how relevant it may be but worth reporting
again, not sure if they're going to even use this method for stable and if not then I may try and figure out what the heckers is going on myself to try and make it more stable so it can be plonked in as some sort of stop gap for folks like myself