Changing the background color of the File-Manager

You can install a "dark" GTK theme. Many popular ones are Numix, Delorean Dark, Ambience Dark, etc.

My personal favorite is delorean dark and here are instructions for installing that theme:

mkdir ~/.themes
cd ~/.themes
wget http://fc04.deviantart.net/fs70/f/2012/344/0/4/delorean_dark_theme_3_6_g__vs_2_56_by_killhellokitty-d5nnoxt.zip
unzip delorean_dark_theme_3_6_g__vs_2_56_by_killhellokitty-d5nnoxt.zip

You will need a tweak tool such as unity-tweak-tool and or gnome-tweak-tool to apply the theme.

sudo apt-get update
sudo apt-get install unity-tweak-tool gnome-tweak-tool

See my similar answer here.