This is a very obscure background. There were actually more colors, but it was animated in such a way I couldn't understand. I guess this works if you use very fast scrolling upwards or downwards. This seems to be the only BG, besides Snot a Problem that's 512x512. The archive has:
-a .map16 file that has over a quarter of Map16 consumed.
-a .pal and .palmask file. This uses palette 1.
-a sample file with BG.
-only 1 ExGFX file and it seems to use the least tiles possible.