The problem is with the way you reference the static files. For example:
<link href="css/bootstrap.min.css" rel="stylesheet">
That tries to load the stylesheet from a path relative to the current URL. That will only work when you're in the root of your site; as soon as you access a different controller or action, the relative path will stop working.
As shown in
the documentation[
^], you need to use app-relative paths:
<link href="~/css/bootstrap.min.css" rel="stylesheet">