Yes.

To change the background, click on the menu button in the upper left. Under map options, select background and choose the one you want.

To make it your default, in the same place (map options), select other options at the bottom. Then click on "Set Current Map as My Default." This will make the map the default whenever you bring it up on that specific computer. (It is tied to your browser, not your account at the moment - I'm going to change that someday.) Anything showing or not showing on the map will also be included in the default map, so make sure it looks exactly like you want it to show up.