Edit html source code?

I want to do some modifications on the HTML in PunBB. Just simple stuff, like changing the user info in topics to a smaller font, and stuff that's not possible by just editing the CSS. Normally this would be really simple, by just adding a new class somewhere in the html, then define it in the CSS.

However, I find it so hard to edit the html files... I open viewtopic.php, and there's just a big php mess! It's almost impossible for someone with no php knowledge to do simple html editing.

I remember I had no problems doing this in earlier PunBB versions... what happened?

So is it just me, or doesn't PunBB want me to edit the html? I installed PunBB again on my new website because I remember from a few years ago, it was so simple it was perfect. But since I like to customize the forum layout a bit, it seems like I've made the wrong choice...

Can someone give me a few hints on how I should edit html, like if I wanted to give a new font class to the user info in a topic? (post count, location, etc).
Or should I try something else like phpBB if I need to customize the forum design beyond simple CSS editing?