I somehow did something to the PERSONALIZE screen so that now when I go to it to try to change something it only stays visible for about 5 seconds, with a whirling blue circle, then goes black for 1 second and returns to the desktop. I need it to stay visible so that I can work on setting up a theme for my desktop.
At the bottom of that window there's a place that says SLIDE SHOW. There used to be a picture there, now there's a black screen which I think is what is causing the problem. I don't know what I did to make it a black screen there, and can't fix it because it disappears too quickly.