Skulls and skeletons are seen as symbols of:
Death and mortality
Impermanence,change and transformation
Nonconformity, free-thinking, and rebelliousness
Toughness, courage, bravery or indifference to death and danger
The skull is also a symbol for intellect,rationality,and a vessel for the soul.
Whatever it might mean to you,or if you simply enjoy the aesthetics for no particular reason,be welcome!