To change the lightbox background, you can use custom css. Please add the css to your templates css. Many templates have a custom.css file or custom css option where you can add this, or you can add it to your templates main css file. This is done in Joomlas template manager. Please add css like this:
You can edit the color values in the css. 255,255,255 is a rgb color value, this sets the color of the lightbox background.
0.8 is the transparency of the background, it can be set from 0.0 to 1.0.
#999999 sets the the lightbox text, it can be any hexadecimal color.
If you need assistance with css changes, please contact ignite gallery support.