Dark, gloomy, bleak, sad (to prevent any unintended negative connotations)
Nature, landscapes, animals, scenery (to avoid any unrelated backgrounds)
Sci-fi, horror, fantasy (to prevent any unintended creepy or frightening elements)
Violence, weapons, blood (to ensure a non-violent and non-threatening scene)
Monochromatic, dull, lifeless (to avoid a boring or uninteresting composition)