J.D. This is already possible. I have a tutorial on this here.
@Heath H-M , how did you do it?
A while ago I did it by having an html element on the page and a snippet in the tracking&analytics section (it was a premium account), then I sent a message from the page to the iframe and from the iframe to the snippet.
It worked well, but It's not super convenient and it's for premium accounts only.
It just uses a third party single colour pixel API that can be used to change the document colour.
@J. D. The method you mentioned is how I had to originally do it for one of my clients.
They couldn't work out how to add extra customization, so I played around with some APIs and found this.