They have a background called deep blue sea. You may like that. It is darker on the bottom than on the top. A gradual lightening, but not too much.
One comment I had for you is that appearance wise, I put my thermostat on the top right of my fish tank, but I stick it on the side wall not the front one. I think it would look better that way. Mine is the same as yours and very easy to read like that. Just thought I would throw that suggestion out to you.
Looks pretty good. Your fish would love a slate cave or for a long project buy some pvc that is big enough for your fish to swim through easily. I have 3" PVC. (You can get curved PVC to put in the corner of your tank) and you could silicone gravel and slate to that. I have one in my fish tank and I like it. Buy aquarium safe silicon though. Make sure to do a search here on slate and bogwood if you want to go that way. Some measures need to be taken with certain materials.
Lastly, if you did want slate, look in a phone book for a local rock place. We have tons of them around here as there is lots of mountains, rocks and granite and I got my slate for free. Don't know how much a LFS would sell them for, but a lumber store or someplace like that is bound to sell them for cheaper than a fish shop. Well, that is all. Good luck with your tank.