Custom CSS basics

I’m reasonably confident the CSS is valid as I’ve used the code from this WPBeginner tip.

body.category-tv { 
background-image: url("http://example.com/wp-content/uploads/2017/03/your-background-image.jpg"); 
background-position: center center; 
background-size: cover; 
background-repeat: no-repeat; 
background-attachment: fixed;
}

To get the classname I used Inspect Element (in Firefox).
There seem to be two nested statements that highlight the right area when I roll over them with the mouse.

<div class="col-main">
  <div class="entry-content">

I’ve tried replacing category-tv in the above code with both (as well as using a link to an image file on the localhost site) but it makes no difference at all.