Appearance on SlateOS
One command does the whole desktop (windows, panel, terminals, notifications, launcher, GTK apps), live, and remembers the choice:
slate-theme dark
slate-theme light
slate-theme wallpaper /home/me/Pictures/sea.jpg # PNG, JPEG or WebP; absolute path
slate-theme wallpaper default
slate-theme transparency reduced # solid panels, dock and menus ("reduce transparency")
slate-theme transparency on # frosted translucency (the default)
slate-theme status # {"theme": "...", "wallpaper": "...", "transparency": "..."}
Tier: reversible (the previous choice is one command away; say what it was).
Wallpaper requests: find the file first (ls ~/Pictures, find ~ -iname '*.jpg'); if the user names a picture that is not on disk, say so rather than guessing. Do not download images unless asked. A picture the user just took or downloaded is usually in ~/Pictures or ~/Downloads.
Verify with slate-theme status and, when the user wants to see it, a screenshot. The change is immediate; nothing needs a restart.