NextGEN Gallery 2.0.7 introduced a fresh new way to implement custom CSS for your NextGEN Galleries.
Originally users had to create a file to include within your theme directory. While this was fairly simple to do, the file could easily be removed with a theme update.
It was originally not included within the plugin for the same reason.
That is why we implemented the new method of custom CSS.
As you can see, the new custom CSS feature will add a file to a folder inside your wp-content directory. Using this method, your custom CSS will stay in place with every theme and plugin update.
Custom CSS will not work for NextGEN Pro iframe based galleries.
Now go have fun with CSS,