Customize the bubble?

Hi,
I’m trying to add some css to the info-box of a POI (like round corners, different background..) but I can’t seem to find the right css class. Which one is it, is there any??
Thank you for your help. I really like the plugin.
Hanka

http://ww.wp.xz.cn/extend/plugins/mappress-google-maps-for-wordpress/