Sitemap XML broken by custom styles

[cfoellmann]

We need to properly include styles the WordPress way.

Temporary fix by Aaron via mail:

public static function customStyles(){
if( defined("REST_REQUEST") && REST_REQUEST == true ) return;
if ( in_array( 'Content-type: text/xml;charset=UTF-8', headers_list() ) ) return;

But that is just a hotfix