I just answered this in another thread. Here's what I wrote:
I figured out the answer to this question about nextgen-gallery with much digging. You will need some familiarity with CSS to change the colors. I'll try to describe in detail what I did.
First, in the wordpress admin area, go to Gallery/Style. on that page, look for line 60 and change
background-color:#ffd2ad; or whatever hex color you want. That Style editor has a hex color tool you can use to figure out your choices.
This will change the color of the background around the albums.
Then look in the same line 143 and make the same edit. This will change the color of the background around the albums.
Lastly, find wp-includes/js/thickbox/thickbox.css (I had to FTP it from the server to my local drive, edit it with Dreamweaver and then send it back to the server) and edit line 49 to change the hex code.
While doing the above, I looked for--but could not find--a way to make the opacity of the overlay darker so less of the screen would show around the edges of the NGG gallery. I found on line 34 of thickbox.css this code, but changing the opacity from 75 to 95 didn't make any difference.