I haven’t tested this part, but to install it on the system rather than testing, I think you can just edit the Metadata.desktop to give it a different name (so you’ll know which is which) through the Name field, including the one for your language if applicable, and copy the whole directory to /usr/share/sddm/themes.
Wow, seems like reddit banned me for some reason, and I have no idea why. Thanks for pointing it out to me. I have registered an appeal, let’s see what happens. Shame, I had some guides and technical support on there that might have been useful for people in the future.