If post by specific role different css to everyone

You can update the post classes depending on user role to include an additional selector which add your custom styling i.e

function add_contributor_class_to_single_post( $classes ) { 

    if ( is_single() ) {
        $post_id = get_queried_object_id();
        $author_ID = get_post_field( 'post_author', $post_id );
        $author_data = get_userdata( $author_ID );
        if (in_array( 'contributor', $author_data->roles)) {
            // add post class
            array_push( $classes, 'user-contributor' );
        }
    }
    return $classes;    

}   
add_filter( 'post_class', 'add_contributor_class_to_single_post' );

Then update your css to:

.type-post.user-contributor { border: 10px solid #fff }
.type-post { border: 20px solid #000 }