Click on the PageSettings icon of your Page. Then, Add > New Item

In the search box, look for Page-Stylesheet-file.

Create your own custom CSS file called mystyle.css.

Put it in C:\inetpub\wwwroot\hkex\Website\styles
You can verify this location in your Sitecore Rocks explorer by hitting up Website\styles

Then, insert this into the location of your style sheet:

Reload your site and you should see that the background has turned red.

You can also right click on your web page and select “Page Source”. Once the HTML source comes up, search for your file name mystyle.css to verify that the page has loaded it.