This page consists of screenshots from the Custom CSS Screen in Google Maps Bank WordPress Plugin.
This gives an idea about how to add customized css.
Custom CSS : In this field, you would need to provide CSS code manually to add extra styling to the layout of Google Maps. This CSS style will override the plugins css and provide you the ability to customize the basic layout of Maps.
Once everything is being setup, you would need to click on “Save Changes” button located at the bottom of the page to save the changes.