Please provide a URL illustrating the problem.
The infowindow inherits most of its styling from your blog so it’s probably something in your theme. You can see if that’s the issue by switching to a standard theme for a minute, like 2012.
You can change some infowindow styling in ‘mappress.css’ (just put the changes in your theme’s ‘styles.css’). The classes are ‘mapp-iw. Much of the style comes from Google itself, like the shadow, corners, tip, etc.
If you use the ‘infobox’ bubble type instead of ‘infowindow’ (you can change it on the MapPress settings) then you have more control over the CSS. It is styled using the ‘mapp-ib’ classes.