If you want to target modern Opera versions (9.0-9.5b2+), you may use this code as a wrapper (taken from this site):
@media all and (min-width:0px) {
head~body:lang(en) [place Opera-specific css here]
}<html lang="en">
UPDATE 21.06.2009: Now css3 media queries have got support in Fx 3.5, so the changes are visible in that browser too. Use the solution proposed here.
