Done.

Through the url, since you can't do this by default through the editor yet, just add to the following to the start of the equation: " \bg_######"

where each # is hex value from 0 to F.

This follows RGB standard, so first two ## are Red, next two ## are Green, following by ## for Blue.

Example:

• gif.latex?\bg_ff0000 \sin(x) - a red background
• gif.latex?\bg_0000ff \sin(x) - a green background

Basically the implementation is as you suggested. Will in time do something similar for the foreground.