When I set my design background to black and save it, it automatically goes back to white.

This is no setting you can save - it's only a helper to make the designs more visible. If you need the background changed on the front-end you'll need to use a bit of custom CSS, something like this:

.mCustomScrollBox.mCS-light.mCSB_vertical.mCSB_outside{

background-color: #000000;

}

you can add this via the UI & Layout composer, custom CSS tab.

Thanks! I thought I was able to save it. I'll try the custom css.