It depends on your setup! However you can do it with style sheets like this...
Code:
<STYLE>
BODY { background:#000000; }
</STYLE>
With the STYLE tags it has to be in the .aspx file, but you can put it in a seperate file aswell.
You say that you can't do it with the same logic as you used to. How is your setup? You have an aspx page, on which you have a BODY tag right!? Or?
Hope it helps
Jacob.