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.

Thanks in advance

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