SecurityDog
If this happens to you, can you check if both files exist?
If you look at the source code of the page, you'll see two elements like this in <head>
<link rel="stylesheet" media="not (prefers-color-scheme: dark)" class="nightmode-light" href="<HOST>/assets/forum-ID.css">
<link rel="stylesheet" media="(prefers-color-scheme: dark)" class="nightmode-dark" href="<HOST>/assets/forum-dark-ID.css">
Or you can run the following in the JS console (still developer tools) to get the links to the CSS files.
$('.nightmode-light, .nightmode-dark').map((i, o) => o.getAttribute('href')).each(console.log)