I see nobody's answering. I searched boards a little bit, and found some useful tips of Michele at
this topic.
Basically, I removed all font sizes from the document template except for the body part in css declaration. I put font-size: small there.
So it shows up ok in Opera and Mozilla, but in IE, I still have some big letters showing up, like number of topics and posts in BoardIndex. Also, when viewing a topic, body of a message is all big letters. It is annoying, especially because of the fact that most visitors use IE.
So I had to put back old template with fixed font sizes declared on various places in template.php. I would really like to make this "remove all font sizes except for the body part in template.php" thing work.