Widget gallery with mouseover effect

Hello.
I want to have a widget gallery and to give it more emphasis with a mouse-over effect.
I read the article: Fade Your Thumbnails On Hoverbut I couldn’t understand where to put the CSS snippet.
I tried to put before the “.ngg-gallery-thumbnail” the id of the widget but it didn’t worked.

My site is paiones.gr
paiones.gr

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