Change Z index of Compose pop-up in Messages plugin

I have the Private Messaging plugin installed. On the Inbox page, when you click on the blue Compose button it pops up the compose box, but it appears behind my site's menu bar. Is there some CSS I can add to my theme or somewhere else that will change the z index of that pop-up box so that it is always on top?

https://thatchurchconference.com/members/inbox

Here's a screenshot of it: https://www.dropbox.com/s/pkb0n6j7s9xkrf9/Screenshot%202016-04-14%2017.33.29.png?dl=0