Increasing gap between the rows

First of all …. your plugin gave me some hope that I can design my own stuff… good one 🙂

I am new to WordPress or let’s say I am new to everything so just have one minor question for now.

I am displaying the images in default theme with short code:
[nggallery id=1] displaying 12 thumbnails on each page width 200 x 200. Space between the 3 columns is fine but space between rows is narrow so I would like to increase it.I looked in many different files and my guess is it is one of the CSS files. Would it be possible for anyone to guide me where I will find these parameters.

Thank you in advance.

http://wordpress.org/extend/plugins/nextgen-gallery/