Changing background color

It looks like this video gallery appears within a div class called “entry-content”

On line 332 of your style.css file, you’ll find this code:

.entry-content {
background: #ffffff;

simply change the #ffffff (which is white) to the color of your choosing.

You can indicate “black” by writing “#000000” or just “black”.

Here’s info about using the developer tools in Google Chrome to inspect elements on your page to work with the CSS.

https://developers.google.com/chrome-developer-tools/