Custom Background Css

Hi,
I would like to change the background colour of the lightbox I am using on teh following website:
http://matthiasviertel.com/portfolio-item/blandine/

Example: http://imgur.com/bxGsZ0x

I am using the plugin Fulscreen Slides: http://abcfolio.com/help/wordpress-plugin-fullscreen-slides/
The background of the fullscreen Lightbox is greyish and I would like to change it to black.

I know that this has to be done by customizing the “fullscreen-slides.css.” of the plugins. I have already tried to change it by changing “transparent” to “black” but it did not work.

Does anyone know how to fix this?

Best regards,
Fabian