Adding extra popup themes is broken

The 'popover_available_styles_directory' filter overwrites any existing themes when hooking into it. This can be fixed by changing line 135 of /popover/popoverincludes/includes/functions.php to:
function P_style_dirs( $styles = array() ) {

Apart from that, good plugin - the unique id's required are perhaps a bit too general and should possibly be prefixed with wpmupopup- or something similar (#messagebox and #closebox) but was surprised at how easy it is to add in a theme.