I had the private variables because I'd never attempted to create a background like that before, and at first it was the only limiting factor that prevented the blocks and lines from spawning infinitely. Evidently it doesn't need them with the destroy actions I inserted.
The first three versions of the background (before I inserted the destroy actions) actually crashed my computer. I took me 6 hours to do that, and you fixed it's problems with two suggestions. Thanks!
Also, what do you think of the background? Would the blocks be better larger, or smaller?