Create a new file at the following location:

blocks/content/templates/wrapper.php.

Enter the following code in the new file:

<div class="content-wrapper">
<?php 
$bvt = new BlockViewTemplate($b); 
$bvt->setBlockCustomTemplate(false);

include($bvt->getTemplate());
?>
</div>

Save the file and go back to your concrete5 site.

Add a new content block and click on it after you've hit Update and select Custom Template.

Select Wrapper and update again.

When you look at the HTML code generated by concrete5 you'll find an additional DIV with a class called content-wrapper.

You can use this class in your CSS file called main.css. Add a rule like this to change the background color:

.content-wrapper {
   background-color: silver;
}