In order to completely replace LWT with this new Theming API, we’ll need to implement a number of user stories whilst making sure that we 1) keep existing themes available and installable from AMO on newer and older versions of the browser and 2) provide a seamless experience for users who currently have a LWT installed and upgrade to the browser version that will support WebExtension Themes. - As a user I’d like to be able to install deprecated LWTs from AMO on older and newer versions of the browser. I’d like all these themes to be dynamically converted to the new package format upon browser upgrade and right when I install a LWT from AMO. - We should replace ‘lwtheme’ to be removed from the codebase and/ or replace them with ‘theme’ named equivalents. This also counts for the CSS(-selectors). - For backwards compatibility, we should continue to maintain the lightweight theme image optimizer code to make sure that header images from old lightweight themes are optimized. Images in these new themes will not have the same dimension requirements of old lightweight themes, and thus will not need an optimization step per machine. We can have online validation during upload of the theme to make sure that images are specified with a standard DPI and not exceeding some large dimension. - As a user I’d like all the existing LWT-specific unit tests to also pass using the new setup, thus making sure we don’t regress its existing feature set.