Do you mean on your web page or in the game itself? In the game, go to the layer's panel and click on the default layer in the game. This layer is set up with a solid background, while all other layers are set up by default with transparent backgrounds.
Once you have that default layer selected, go the the Properties panel and you'll see a field for background color that you can change.
If you mean the "envelope" or "bars" around your game that appear because of scaling, they should automatically be black. If they are not, I'd report it as a bug. (In appmobi right now I'm encountering a white bar on the rhs that is due to some sort of scaling problem, but I assum you aren't using appmobi.)
If you mean on the web page, that is just like you'd change any web page.