I think you need to refresh some basic skills:
<link rel="stylesheet" href="css/styles.css">
This URL is relative to the current page, not to the root of your Hugo site.
To repeat myself from our earlier conversation, I think you would be better off starting with something that works. Your test site is… a bit of a mess.
There are some great resources here:
https://gohugo.io/getting-started/external-learning-resources/