FB Purity lets you set a background image for Facebook and use a custom Font
The safe, free and top rated FB Purity web browser extension lets you easily customize the look of Facebook:
Install FB Purity via the official FB Purity website
To set a background image:
1) Open the FBP options screen by clicking the FBP link in the top nav bar.
2) Click the "Fonts, Colours and Design" heading on the options screen
3) Enter the full URL to the image you want to set as your background image in the "Background Image" text box. The URL must include the image file extension e.g https://somesite.com/images/somepic.jpg
4) Tick the "Set" box next to the "Background Image" box
5) Click the "Save & Close" button.
-----------------
To set a Custom Font
1) Open the FBP options screen by clicking the FBP link in the top nav bar.
2) Click the "Fonts, Colours and Design" heading on the options screen
3) Choose a Google Web Font from the "Web Font" drop down box
4) Tick the "Set" box next to it.
5) Click "Save & Close"
FB Purity has hundreds of options for cleaning up Facebook, letting you hide the junk you don't want to see. It also alerts you if any of your friends un-friend you.
FB Purity is safe and has been reviewed and recommended in The Washington Post.