I have Ubuntu 14.04. The background color of the File-Manager. White is too tiring for my eyes. Is there any way I can change this to something dark, maybe gray or black?
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