Actually, in my opinion there's nothing special needed for this -
real graphic designers know better than to set the background color to
something that will munge with an icon or other inlined image on the screen.
As far as being downwards-compatible for users using less than 256 colors
in their graphical browser... I don't know, but that's an issue I'd
rather leave up to the client. All graphical apps I know of are at least
capable of 256 colors, so I don't think it's unreasonable to design for.
In Dave Raggett's HTML 3.0 presentation at WWW'94, he suggested that one
solution might be to have a background bitmap that gets tiled - thus more
complex blackgrounds are possible, something we'd really like.
I have a whole HTML wish list of items that could only be implemented in
the spec, but I'll save that for www-html....