How to change the background of the cards from white to black for dark site?

Which template style? and which background exactly. A link to your site or a screenshot will help.

I've got Style 7 and want solid black #000 background. I've tried even targeting CSS. It looks ok in the backend but front end always still renders white background.

Here is the front end. https://wordpress-1571916-6456151.cloudwaysapps.com/

I'm not seeing the CSS rules on the front end. Make sure to clear all cache plugins.

Does it look correct in the Preview?

Thank you! It was a cahcing issue. By the way what does the field Background color do on the settings panel? I couldn't find any documentation and changing it didn't seem to do anything. 

Great! Glad you found the problem.

The background color is what I used to set the ones in my screenshot to a black background.