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.
Current Phone ~
Samsung Note 20 Ultra 5G SM-9820 _256Gb SnapDragon 865+
Samsung One Ui 2.5/ Android 10.