Although I will not directly answer how to style the information tabs since each site has different requirements, I will give you some information that might help.

The following list of elements are what is used in the rendering of the information popups:

markerName - This is the MarkerName or title that by default is underline.  

markerDescription - This can be used in describing the markers description text.  The entire text is wrapped within this DIV.

gmapsdirections - The form used to obtain directions to a given marker is wrapped within this element.

directionstitle - On the Driving Directions tab, you will see text that reads "Get Directions to ...".  Use this element if you want to style that text.

gmapstab-image - This class is used for the DIV element that wraps the table that includes the image and detail and the image only information.

So what do I do now that I know that information, for this, please goto a CSS tutorial.  http://www.w3cschools.com has a pretty good tutorial on CSS.