Loading bbPress CSS file only on forum directory

The styles are enqueued in the function enqueue_styles() inside the file /wp-content/plugins/bbpress/templates/default/bbpress-functions.php.

It’s a matter of using is_bbpress() and wp_dequeue_style. Only one of the styles is enqueued, but here we’re removing all 3 possibilities.

add_action( 'wp_enqueue_scripts', 'bbpress_enqueue_wpse_87081', 15 );

function bbpress_enqueue_wpse_87081()
{
    // Check if bbpress exists
    if( !function_exists( 'is_bbpress' ) )
        return;

    if( !is_bbpress() )
    {
        wp_dequeue_style('bbp-child-bbpress');
        wp_dequeue_style('bbp-parent-bbpress');
        wp_dequeue_style('bbp-default-bbpress');
    }
}