The partition thing was part of the installer. I used the suggested partitions and when that didn't work, tried resizing the suggested partitions. When that didn't work (can't resize an efi partition apparently), I made the partitions myself and it was the same thing.
I was already a bit annoyed by the aspect ratio and needing to access a hidden menu to boot with an Nvidia card so I stopped trying after that.
I might install those just for fun but I'm pretty happy with the default plasma dark theme!
Aha, that works too! I'll edit the post to suggest this to future readers