To change themes completely to a different color in Ubuntu should be trivial and take only a couple of clicks. It seems though that there is a bug that prevents the background screen from changing colors. I'm going to go through the process of changing everything to a different color.
The first place we need to go is the "System", "Preferences", and "Appearance" menus. Select the background tab. From there I pick the solid color, and then select an alternative color. (Blue, in my case) You can of course download a wallpaper and use that as well. Then select the theme tab to choose a non-brown theme.
The next place we need to go is "System", "Administration", and "Login Window" menus. Select the
local tab. Once there, I chose the Happy Gnome theme and changed the background color to blue.
That should be the only steps required to get rid of all the brown. However, while logging on, the brown background still persists. After searching for a while, I did find a fix.
The script /etc/gdm/PreSession/Default is at fault.
You can download a version that fixes the background color like this:
Then just copy that to the original directory, restart your session, and the brown will be gone.
Because you shouldn't just blindly copy files into your system without knowing what's in them, here's a comparison of the two files, showing what's been changed. I used the program TkDiff to display the output you see here. The original file is on the left, while the new file is on the right.
That's all she wrote.