Change $9511 in the ROM to whatever colour you like.
I found it using my typical method of debugging in FCEUX. I went to $3F00 in the PPU to see the background colour (36) and did a write breakpoint. This took me to $305 in RAM. So I did a write breakpoint to that, and it brought me to that section of the ROM. It's a routine that fills the palette table with all colours, but the background colour is only needed once. Nevertheless, it fills each BG colour space with 36, but I discovered through process of elimination that only the write to $3F10 actually matters, so just change that one byte and you have a different background on the title.