Need to move the Private Messaging Send New Message Box Down


I just installed the Private Messaging plugin. When the Send New Button is click on, the modal box opens, but it is behind the header bar.

I added this CSS to the theme's custom CSS.

left: 50%;
top: 30%;
width: 659px;
margin-left: -329.5px;
display: block;
position: fixed;
z-index: 1000;
max-width: 659px;
opacity: 1;

This moves the box down to where it needs to be, but it is always open and it cannot be closed.

Could you take a look at it and let me know what I'm doing wrong?

Here's the URL:

Having the same issue on the Inbox page:

Any suggestions would be appreciated.