[fel64]

No, it isn’t.

Stylesheets are what determines the look of a forum. These are the stylesheets the WP.com forums use:

http://wordpress.com/forums-theme/wpcom/forums.css

http://wordpress.com/wp-content/themes/h3/style.css?1169109548

http://wordpress.com/wp-content/themes/h3/hack.css

I do not think they would mind if you use them, as long as you give credit. You could ask someone.

Or, instead, you could start a new theme, edit your new style.css file and try to replicate what they have by try-and-error.