Adjusted color of background, fixed navigation panel text color, and some more.

Same as original author's (Zebouski) permission, as long as they don't change the permission of the original theme. You can do whatever you want with this, I only changed a few things in a couple of hours.

Oh and, it turns out that personalhamster@userstyles was ahead of me in making these tweaks. The adjustments are similar but not exactly the same. Here's their style: http://freestyler.ws/style/142827/google-docs-dark-sepia-mode

Finally, a main page Sepia mode is commented out since the printer actually prints the background color. You can enable it by un-commenting that part, and disable it again when you want to print something.