The following works for me in Chrome, and doesn't prevent the menu from going outside of the viewport:

    select, option {
        background: red;
        font-family: 'comic sans ms';
    }

Yeah, it only works in Chrome

I think that you can compose arbitrary images that way using an image-like font (using various Unicode chars and assigning to each a small tile of the full image)

Actually I am now curious of how much detail/size/colours/animations you can fit in a single letter.

If that's all your changing, then why can't you make do with the system default?