As per your clarification.. this is how you can do it..
<html>
<?php
$homepage = file_get_contents('https://auspuff-schuelerzeitung.de/');
echo $homepage; //you can strip the closing <html> if it bothers..
?>
<link rel="stylesheet" href="https://stackoverflow.com/questions/40294645/my_changes.css" type="text/css"/>
</html>
5
solved Add CSS to existing website