I'm using Plain TeX and trying to create a box that has a shaded background, containing a mixture of both mathmode and textmode TeX. It would be great if I could add colour to the background (i.e. like black text over a pale yellow background) but grayscale would be fine, as well.
I've looked around and I've found some implementations of this but I have not found a flexible Plain TeX package. Is there not one? Perhaps I don't know what to search for.
Thanks for any help.