frontend / css implementation

Hi reinoud26, sorry for the delay,

all the plugin css styles are in docu/assets/css/frontend.css

about adding last updated time, you can use WordPress native the_content filter:
https://codex.wordpress.org/Plugin_API/Filter_Reference/the_content
something like this will work, add this code to your theme functions.php

function custom_docu_add_last_modified( $content ) {
	global $post;
	if( get_post_type( $post ) == 'doc' ) {
		$output = '<p>Last modified: '. get_the_modified_date() . ' ' . get_the_modified_time() .'</p>';
		return $content . $output;
	} else {
		return $content;
	}
}
add_filter( 'the_content', 'custom_docu_add_last_modified' );

Thanks.