04e2ae3af3
When a theme doesn't specify popup colors, it makes sense to follow the system-preferred color scheme. Refactor theming attributes so that we can distinguish this. Instead of a single [lwt-popup-brighttext], we get lwt-popup="dark"/"light", and handle it appropriately. Same for the other relevant attributes. Refactor sidebar theming (which already did this tri-state thing via lwt-sidebar and lwt-sidebar-brighttext) to do the same using a single lwt-sidebar attribute, and update the docs. Differential Revision: https://phabricator.services.mozilla.com/D190919
