Yes, that is possible. See
<http://my.opera.com/Rijk/blog/2004/05/25/using-browser-css-and-user-css-in-opera-7-5-for-non-geeks>
for some help. It is a bit old, in recent Opera versions you can set a
specific user style sheet for a single web site such as microsoft.com.
--
Rijk van Geijtenbeek
Opera Software ASA
http://my.opera.com/Rijk/blog/
"The most common way to get usability wrong is to listen to what users
say rather than actually watching what they do." - J.Nielsen