You can choose to engage the default theme @JulsG that came with the phone out of the box.
As far as I'm aware you cannot turn off themes.
Long hold on a clear portion of your homescreen and go into themes and tick the default theme at the top if your currently using a downloaded one.
Daily Driver > Samsung Galaxy s²³ Ultra 512Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "