When using this on floor arrows, the arrows show as black no matter what colour I choose :/ I didn't try it on anything else.

Hi Amnaa!
It's possible: I wanted to make an "open sea" background, as if the fishes were in there natural environment (I don't like "enclosed" backs). For this, I used the top of an unused theme wall I found in game files, and the tank sand floor for the bottom (and a plain blue at the center because in the deepest tanks the center zone is repeated vertically).
I'm also thinking about two other variants: the varech forest and the coral reef, but I need more time for these ones. They're not just a gradient over two existing game tiles ^^. Enjoy!

I must say, the backdrop of these tanks seems VERY FAMILIAR to me...