It’s pretty easy to change the interior of the infoWindow. Look at file ‘mappress.css’ in the plugin directory. The classes under ‘.mapp-iw’ are all styles for the infoWindow body, title, etc.
Changing the infowindow shape is harder. That HTML and CSS all comes directly from Google. You could try to style it but I think it will be difficult. If you want to try you can use Firebug or a similar tool to see what classes they’re using.