Downside: the levels felt a little linear. I had to restart a few times, but mostly it was "go to the only block you can reach; repeat".
Upside: literally everything else! I've seen "change the background color" before, but usually in the red-to-green-to-blue spectrum (to the point that I nearly skipped over this, for fear it was just another one of those), and almost never as shades of the same color. So it's a new and exciting use of that mechanic, and a really neat interpretation of "deeper". Also, the level design is really good; I loved the trees and pyramids and general feel of it. Good sprite; good music; good sound effects; good for you for getting the vaccine. Just good all around. :-)