Custom CSS for ‘listing-item’ class?

While trying to implement

[display-posts wrapper="div" wrapper_class="popWrapper" include_content="true" content_class="popContent" category="po"]

I couldn’t seem to get display:inline-block working so that the posts would line up into rows instead of just one column.

Viewing the page source of my page showed me that the plug-in seems to automatically create a div with class “listing-item” for each post to be contained in. So I tried adding CSS for this class in my page’s template file, and it didn’t seem to work at all.

Is there a way to customize the ‘listing-item’ class? Do I need to put the style/CSS somewhere else?

Thank in advance