shell-background-is-not-scheme-background

The moment an app shell paints its content panels in anything other than colorScheme.background, every gradient, scrim and fade whose tail converges on colorScheme.background ends on a hard seam — the decoration is still correct, its destination colour is just no longer on screen. Covers resolving one page-background value and handing it to every decoration, the two valid answers (aim at the shell colour, or paint…