What version of bbPress are you running? The 1.0 version has some classes that you seem to be missing there. I don’t have a 0.9 version installed with categories to check this out.
This would get you close, at the bottom of your style.css, but it’s not quite right – it spaces all the table data cells over. You just want the first ‘column’ over, the forum category.
.bb-root td {
padding-left: 2.5em;;
}
I didn’t think the older version had forums as categories – are you using a plugin for that, or are you using 1.0 and just missing some classes for your theme? Maybe that theme was for 0.9 and is missing the markup for 1.0.