Follow-up to the 3.4 colour purge: the sidebar panel still showed Firefox''s dark blue-grey default (rgb(28, 27, 34)).
FoxOne only pinned the --lwt-sidebar-* tokens, which the revamped sidebar (sidebar.revamp) no longer consumes - the panel reads --sidebar-background-color directly. That token is now pinned to --uc-color-base (#282828), with --sidebar-text-color following the palette text colour. The dynamic-theme hand-off (Adaptive Tab Bar Colour and friends) stays intact.