post format background color change?

I’m trying to change the background color of the “aside” post-format. Ideally it’d be a third color choice (instead of the default and “dark”) but I’d settle for changing it to “dark” since I use the a/v posts less frequently. As it is, the lack of a header means it kind of “bleeds into” adjacent posts, visually. A simple background change would fix that.

I’ve been through the css and have found the code associated with “aside”, “audio/video” and with the “dark” post-format. I can’t figure out how a particular post format is designated as “dark” or not.

I’ve looked into plugins that can change individual posts’ background colors but the “background” they change per post is the whole site background, which I’d prefer to leave alone. However, if anyone knows of a plugin that can do what I want, I’d be happy to hear about it.