I inserted your code, verbatim, into a JSFiddle, and the background is red, as you wanted.

The code is not the issue, then. Instead, its a problem of the stylesheet loading properly. Try some or all of the following:

• Check your stylesheet name, and that it is the same in the directory as it is in your HTML code; make sure stylesheet.css exists properly.
• Double-check that the file exists in the right place, which is in the same directory as your index that you are opening.
• Make sure the stylesheet is loading. In Chrome, if you right-click -> inspect element and go to the sources tab, you can see all loaded resources, including your CSS. If it's not loading here, it could be that it tries to load from your cache. You can force Chrome to load sites without the cache by opening the inspector again, clicking on the gear menu, and checking "Disable cache (while DevTools is open)', and reloading your site.
• Check your local apache/webhost server to make sure that it doesn't have any settings blocking the loading of certain files or types of files. This is assuming you are running one and not just opening the file in Chrome/IE. If that is the case, you should have no problems with the file loading.
• Make sure you saved the file. It's silly, but I can personally vouch for the time when I couldn't make my site update, and I forgot to save/update the file I was viewing.

Did you double check the spelling of the CSS file? Can you show us the CSS?

It doesn't look like you are actually linking to the directory of the file, rather you are actually just linking the name. Since your index.html is at the root of the directory you have to specify in the href attribute that the css is located in a folder called styles you would do so like this. html In your case you say that your index.html file is in a folder called Zero with all your CSS files in a folder called styles and you want to link to a CSS file called global.css. You would do that like this html I'm assuming this Zero folder does not actually live on an actual webserver that you are hosting your website on, rather than a folder on your local machine that you are using to keep your files organized while designing before you upload it to the web, because you should always have your index.html inside the root folder.

I invite you to read this article Absolute and Relative Paths

Then we pass to your code:

<link rel="stylesheets" type="text/css" href="/styles.css">

Should be :

<link rel="stylesheet" type="text/css" href="styles.css">

Your styles.css should be in the same folder as your html file.

To verify that you have an error , check Console of your browser,you will be noticed that your file doesn't exist(404 error).

An other way to make your css working is to integrate it inside your page without separation:

Example:

 <style type='text/css'>

    body {
        background-color:#CCCCCC;
   }

    h1 {
        color:#0000EE;
    }
</style>

Your CSS folder needs to be in the HTML_Personal_Site folder.

Edit to expand: Your index.html file resides in your HTML_Personal_Site folder so that's where it begins the search in your path. Currently it's looking for a folder named CSS within HTML_Personal_Site. If you wanted to keep your CSS folder where it's at for some reason, you'd add ../, which denotes moving a level up in your file architecture, to your current path to give you ../css/styles.css.