I never thought of a black background. I was pretty partial to the faded green to blue one
I'm partial to painted on black background myself, but have seen some really good looking bluish ones. But then, my idea of a good background is one that manages to cover up the mess behind the aquarium, and doesn't draw attention from the plants/hardscape/etc. I don't think specific color (black, dark blue, dark greys, green, etc.) is so important, as long as it doesn't serve as an attention draw in of itself, and just serves to provide a good mask(for whatever happens to be behind your aquarium), and backdrop for the plants that you do have.
I've found black spray paint to be pretty easy to apply, cheap, and gives a nice, subdued, uniform background that doesn't draw attention.
The big rocks are holding the wood down. It refuses to sink..
It'll probably sink (eventually). If you don't want to wait, you could remove it, and screw/glue/bolt it onto a slate base - that would probably be enough to hold it down.
On the other hand, the rocks don't look half bad, so if they are long term, or even permanent, I don't see that as a negative.