Are you sure that your HTML page is correctly seeing (linking to) the CSS files? You can check it in Inspect window of your browser. I can see you’re using Bootstrap in your CSS folder, this needs some configuration etc. Have you done it?
Generally if you’re new to web-coding in general, it may be difficult to use tools like Middleman right away, since it’s a conglomerate of many technologies, libraries, tools, etc. Many things to lick.