Problem with selected Desktop color-v-Mojave background

I'm on an old Mac Pro and working in Mojave, which is fine, but for one problem: I can switch the Desktop color from the Mojave sand dunes and mountains to one of my choice, and it lasts about a week, then goes back to the sand dunes. My reason for preferring my color choice is contrast. I can see all the desktop folders and files easier against a solid medium-blue BG than I can against the Mojave image. What do I need to do to keep my color choice on the desktop?