My computer’s theme display is having issues

Hi,
I modified my computer’s theme and it is working fine. Recently, I decided to alter it again but this time, something went wrong. It displays figures that were not supposed to be there. For instance, a rectangular figure shows up in the middle part of my screen with transparent stair steps on the upper left corner. I have tried troubleshooting but it has not worked. I went back to my simple settings but the figures are still there. I have also tried restoring my system but it did not change a thing. What do you think is the problem? What am I meant to do in order to fix this?
