What you want to do is tweak the bbPress CSS. However you don’t want to tweak the copy in the plugin because when you update it will get overriden. Here’s what you’ll want to do.
Copy:
wp-content/plugins/bbpress/templates/default/css/bbpress.css
to:
wp-content/themes/[YOUR-THEME]/css/bbpress.css
Then edit and make tweaks to the bbpress.css as needed to match your theme.