This repository contains the UserContent.css file (to add the newtab background) to be used in unison with the Firefox Theme. Download a release then deposit the files into the "chrome" folder inside your profile folder (if it isn't there, then add it). Also make sure the "toolkit.legacyUserProfileCustomizations.stylesheets" tag is set to true in about:config, to enable the file.
*The profile folder is accessed using about:support