Thunderbird is no longer able to install themes or even open the web page internally. It redirects all pages to firefox, and firefox installs the theme into itself. I have read that this is because GitHub has changed owners and changed the protocols used, however there is no way to install the theme images themselves into Thunderbird, even by downloading the theme into firefox, opening the profile tab and overwriting the current "lightweighttheme" files. The links cannot be "saved as" either. I'd just love to be able to use all those lovely thunderbird themes.