I'm trying to add custom css to the map to accomplish

I'm trying to add custom css to the map to accomplish the result of the attached image, but the the id of the div (map_519207210013_preview) is getting changed when you refresh the page.
http://i.imgur.com/vNXbsO2.png

In which file of the plugin can i add a custom class so i can add custom css to it? I wanna add to the map css (width: 50%; float: left:wink:.

Thanks in advance.