The theme of Risk is world domination through military conquest.
The simplicity is the theme: war reduced to dice, borders, and exponential reinforcement. It captures the feeling of empire-building—endless fronts, overextension, the dice-god's cruelty—without chrome. That's why it's lasted 65+ years.
Variants that deepen theme: Risk: Legacy (permanent board changes), Risk: Europe (WWII), Risk: Godstorm (mythological), Risk 2210 A.D. (moon/water/nuke).