Get background color for Live Preview with Theme Customization API?

Question

This could very well be a stupid question, but I'm working on a WordPress theme (check the link to see what I'm talking about, then read on)...

You'll see the Post Formats icons are surrounded by a white circle, which has a border color matching the custom background color. I know that Live Preview includes background color by default, but I'm wondering how to change this border color with the background color, as they need to match.

Really, I just need to know what setting I need to use with postMessage, or if such a mechanism is possible at all.

Thanks for any and all help! I know it's a quite unique situation, since I can't find anything remotely close to the topic here, on Codex, or Google, or anywhere.