bbPress.org

The reply form border styles is inherited from your theme.

Do you want to remove the topic and forum descriptions too??

This php code snippet will remove the descriptions. Place this code snippet in your child themes functions.php or in a functionality plugin.

add_filter( 'bbp_get_single_forum_description', 'ja_return_blank' );
add_filter( 'bbp_get_single_topic_description', 'ja_return_blank' );
function ja_return_blank() {
    return '';
}

You can try this instead. This hides the “this forum contains … topics… and” description that is blue on top of forums.

add_filter( 'bbp_get_single_forum_description', '__return_false' );
add_filter( 'bbp_get_single_topic_description', '__return_false' );

You can try this CSS for the reply/topic form text areas.

#bbpress-forums fieldset.bbp-form textarea {
    border: 1px solid #ccc;
}

Anything else you want on this site I am sure there has already been a topic about so searching the forums might be best.