Facebook is not officially support for changing its appearance, but some people craze about changing their FB theme and it wants to be look so different,
Here is my workout for the theme changer, Firefox release addons for adding custom CSS for a particular websites, we can do the same for FaceBook.
1, Install Styles addon for firefox
Once installed the addon reboot the firefox is reqired to edit the settings, then you will see the S button on the Tool bar.
2, Add custom CSS for Facebook,
Go to the above url and select once of the style, Here I used FaceBlack (Black Facebook by a3cAnton)
3, Turn the Styles for all websites,
Now goto “Styles” button on toolbar and select “Turn all styles on”
Now facebook is ready to start with new CSS, Checkout other similar CSS for other Social network websites. https://userstyles.org/
Here is My FB looks like.