A full theme is very complicated and needs the same rules as the built-in default theme. Any change to the default themes to reflect things like added toolbar buttons like the forget button or changes in the DOM structure will break all themes that worked with the previous Firefox version and its author need to come with an update. There are development (beta and nightly) versions available that add-on creators can use to test their theme (and extension) if they want to make sure that their add=on keep working. *https://www.mozilla.org/en-US/firefox/channel/