...on krypton. Unless you're using Estuary.
Tested on S905 and Pi3. I've tried at least half a dozen skins and the background only works in estuary, which means otherwise you're seeing whatever's on the page you loaded from. Not too bad if it's a relatively empty page but not ideal.
Examples:
Xonfluence in Jarvis:
Estuary:
Confluence and xonfluence krypton (page visible behind):
Also tried amber, mimic, arctic zephyr, titan and a couple of others - same.
A couple of log snippets:
Confluence
Estuary
Observations from those:
1. There's a reference to 'videolibrary' - line 11 of service-LibreELEC-Settings-mainWindow.xml - that I think should now be 'videos'.
2. Sticking a bg.jpg in resources/skins/Default/media/default makes it perfectly usable, ie that background loads, although you don't get the pretty header / footer / arrows.
3. If I look in estuary there are a ton of references to '<include>DefaultBackground</include>' - can't see them in other skins...?
I've about exhausted my abilities at this point - I'm no coder! - hope someone can look at it.
Edit: just to add the logs are fairly old, still the same on the last Pi3 install v7.90.008 .