Metalloy wrotemy wife got a similar ransomware and I managed to deleted it all and retrieve some files.
first you need to identify what is causing the background to change.
if you go to documents for example you should be able to find a new set of files named similarly that has is not downloaded by you. you'll find it in lots of different folders. if youbdo find it, open search and type in the name, probably you'll see hundreds maybe thousands of files spread out in the PC. deleted them all.
for the retrieval of files, currently I don't have the laptop in front of me but I remember finding a software that might be able to find an previous backup image for the affected files, look it up it works, there is some manual.work tonit it is not automated but it works for most files.
Good luck
I read that only files encrypted Cerber Rensomware v1 are able to be decrypted, sadly no solution for v2, v3 and v4