If you want a certain page to have just one difference, you don't need to copy and paste the whole stylesheet over to a different stylesheet. As long as it is included after (i think) the first stylesheet being included, all you need to put is the changes you want.
If you want to put a small bit of CSS on a HTML page without having a stylesheet, simply put it in
<style type="text/css">
css code here
</style>
I hope this works,
-Ben