If you want to style some of your posts differently you can use Custom Fields to get it done:
In the post page, click on Screen Options (top right) and make sure that Custom Fields is checked
Scroll down to Custom Fields and add a new custom field with a name of your choice, for example ‘css_class’. Add a value to this field, which will be the class name added to this post, for example, ‘gray_back’.
In the template file (index.php, for example), in the ‘loop’ section of the code add a class to the article tag that encloses the post:
<article class="<?php echo get_post_meta(get_the_ID(), 'css_class', true); ?>">
The second value in the get_post_meta function is the name of the custom field.
Style the class in style.css. For example: