I'm using google translate to write this, but I hope you'll understand everything.
borksy has a hack called transparent background (first color in palette).
since bitsy games are html files, we can edit them like a website (add videos, images, etc.)
i edited the css of my game in borksy (a few lines) to set the background to a rainbow gif i found on google
That's basically it.
If you're interested, I can write more about it in discord.