Basex db:open ignores binded .css file

I used the following xquery function to open an xml-File.

declare
%updating
%rest:path("/index")
%output:method("html")
%rest:GET
function b:loadMenu() {
    update:output(db:open("BaseX", "webapp/static/xml-css/Interface/GameTable"))
};

This xml-File has an xslt stylesheet to transform it to html, just like this:

<?xml-stylesheet type="text/xsl" href="interface.xslt"?>

Also, the xslt-file hast a .css stylesheet binded to it for a table, like this:

<link rel="stylesheet" type="text/css" href="style_table.css"/>

My problem is, that the .css file is somehow ignored and I don't know why. The funny thing is, if I just open the .xml file per right click in the browser, everything is perfect. But not, when I open it via db:open in the xquery.

PS: The files are all in the same directory.

UPDATE: Searching with F12, I found this:

Now I am asking myself, why it doesn't find the .css file.