In ‘Tweaks’ -> ‘Appearance’ -> Themes -> Background set to the image u want that’s saved on your local disk. But it replaces the pureOS default wallpaper that appears during launching an App only.
You can check this which also comes with keyboard theme and works perfectly fine in usage but it requires an account on their repo in order to download david hammer’s content and also permission, which you can ping a moderator in here to grant it.