Theme atmosphere: every theme's own backdrop, still or drifting - beta
In Settings → Appearance → Beta Features, turn on Theme atmosphere. Nothing changes yet: what it does is add an Atmosphere control to the colour theme screen, beside Appearance, Applies to and Surface.
Open Settings → Appearance → Color Theme and use the new Atmosphere control. Off leaves the plain background. Still shows the scene and leaves it there. Drift lets it move, very slowly - slowly enough that you notice it only when you look up.
All 40 themes have their own scene, drawn for that theme rather than tinted from a single picture - a coastal theme gets its own horizon, a dark academia one its own. Change theme and the backdrop changes with it, so the two always belong together.
Available on iPhone, iPad, Android, and web at app.stash2go.com