Backpack the Backdrops sprite into your project. Load it up with full screen costumes (but leave the Crossfade Colors costume in place). Then From any sprite: 1. Set the variable TO-BACKDROP-NAME to the exact name of any costume in this sprite. The capitalization (upper and lower case) has to match exactly. 2. broadcast Backdrop Crossfade (NOTE: to make the standard scratch stage backgrounds into costumes, load them on the stage, go to the "backdrops" tab--where the "costumes" tab is for sprites. Drag the images from this "backdrops" tab down onto the Backdrops sprite to create costumes for them.)
Based on a technique from @helloworldbyeworld here: https://scratch.mit.edu/discuss/post/6365784/
