This is a new stylesheet addition in 7.1B4 I believe; just use the style editor and edit the following class:
CP -> Display Options -> Styles -> Edit Theme -> Misc. Properties -> .popup_content

An example of a class for it:
Code
background: #081750;
border: 1px solid #000000;
border-right: 2px solid #000000;
border-bottom: 2px solid #000000;
padding: 4px;
padding-left: 6px;
padding-right: 6px;

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