Thank you for that, Paul.
Not integrated into a website, but the slight problem I had with mine is that none of my members are youngsters with good eyesight and big monitors - the youngest will be 40 very shortly, I'm 55, and most of them are over 60. Generally they find that using "Normal" text size (or "Medium" in IE) is OK for the average web page but nearly all forums (not just PunBB) are too small for comfort.
So, to save them going up a size in their browser just for my forum, I had fished around (a lot of trial and error here - I'm new at this stuff!), found that first bit and changed the default size to 12px. Result - everybody happy except for a few people who were using IE.
I hadn't spotted the second bit though (and probably wouldn't have cottoned on if I had) but by changing that from 68.75% to 75%, this has resulted in smiles from the IE users too.