The default is the Google ‘infowindow’. The main styling classes are:
mapp-iw
mapp-title
mapp-body
Note that styling for the infowindow is limited, since a lot of it is set by Google (for example the shape, the shadow, etc.). If you use Firebug or IE Developer Tools, you can see the Google classes, they’re injected into the document header when the page is loaded.