I spent years playing only Go, and I still consider it the greatest game ever devised by humankind. (In fact, of all the games in my collection it is my only '10'). It has the simplest ruleset of any game I can think of, and its depth is boundless. And, it has blatantly no theme. Even Chess, the other king of abstracts, has more theme (Kings! Queens! Knights! Bishops! Pawns....). And even Go has Life and Death. It seems even in the most themeless of games, some relationship to existence is unavoidable.