Thanks for playing and for the bug report. There's lots of stuff I plan to fix after the jam!
Oh, it's simple! When you edit the theme (the button right next to the PUBLISHED status), upload a background picture (under the IMAGES heading), set as Repeat: Both, Align: Center. The picture I used for the background is 2000 by 360 (as shown here). Feel free to use my image as a template.