Let me first make sure I understand what you mean. I believe you mean
that you have a generic link in your pages to an external stylesheet,
along the lines of ...
<link rel="stylesheet" href="style.css" type="text/css">
... and that it's presumably somewhere in the head. You now want to
override the background colour of something. I'm not sure whether you
mean the actual BODY tag, or just some tags within the body.
If you mean the body then all you have to do is after you've put the
external stylesheet link, put this in your head.
<style type="text/css">
body { Background: red; }
</style>
This will overwrite the stylesheet's body background property, but not
the rest of them.
If you are wanting to change other elements then I'm not sure how you
wish to change them. If you have for example ...
<input type="button" class="buttonclass">
... and you want to overwrite the background colour, then all you have
to do is add in a style attribute.
<input type="button" class="buttonclass" style="Background: red;">
Hope this helps, if not then I'm afraid I've misunderstood your request.
Fred Basset
(e-mail address removed)