because its MIME type ('text/plain') is not a supported stylesheet MIME type, and strict MIME checking is enabled code example

Example: Refused to apply style from 'http://localhost:3000/css/styles.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.

app.get('/style.css', function(req, res) {
  res.sendFile(__dirname + "/" + "style.css");
});