In the twelve theme I would like to change the background of the
thumbnails window so that it is no longer transparent and it has a
specific color.
I tried putting this in the body but it seems to have no
effect.

.galleria-thumbnails-container{ background: black; opacity: 1 }

The discussion is now public.

Cal, go into the galleria.css file and search for
.galleria-thumbnails-container

Play with those settings and add settings to get the effect you
desire.

My Twelve trail slideshow is at the bottom of this page
http://www.dorsetdog.com/galleria-slide-trial-in-google-sites

If you use Firebug in Firefox web browser, it will help you to
identify which area of galleria is called what to help you change
the right css code

I found out the background for the thumbnails container comes
from the bg-png file in the themes/twelve directory. You can use a
photo editor to fill this bg-png file with a solid color to change
the background for the thumbnails. I assume that if your photo
editor supports transparency for png files, you could also create a
level of opacity other than 1.

In my galleria.twelve.css file is

.galleria-thumbnails-container { position: absolute; top: 0;
left: 0; width: 0; height: 0; background: #ffc; opacity: .9;
z-index: 2; }

This background:#ffc; code sets the background colour for the
thumbnail container on the Twelve slideshow at the bottom of this
page http://www.dorsetdog.com/galleria-slide-trial-in-google-sites

Thanks Mark. Your solution works great. But I wanted to leave
the galleria distribution files alone, so I modified my own html
file to accomplish the same thing. I just put this in the head of
my html file:

body .galleria-theme-twelve .galleria-thumbnails-container {
background: #eee; opacity: 1; }