[Renate]

Eink builds tend to do goofy and strong-armed things with wallpaper.
Your friendly neighborhood wallpaper setter will set /data/system/users/0/wallpaper correctly. You can pull that file and check (it's a PNG).
Unfortunately, the Glow '21 never renders that as background, it uses black. Period.
(Semi-unrelated: The Poke3 lets you set wallpaper but it always renders background as white.)

You can try my simple wallpaper setter in my sig.
If you load it on a phone you can see how the background in the app itself changes instantly.

Edit: Also, the Glow '21 doesn't respect wnd.requestFeature(Window.FEATURE_NO_TITLE)