This has been an issue for me for a while now. When you use Dark theme the backgrounds gets darker and same goes for Light theme as the backgrounds gets frustratingly too light.
using dark theme only and didn't notice that background looks any darker than it is supposed to be, as matter of fact it seams a little bit brighter but I can't confirm that 100%...
This is obviously done on purpose so the dark text like the clock is more visible. However it results in very washed out wallpaper which destroys it's purpose.