Some say the world will end in fire
Some say in ice
From what I've taste of desire
I hold with those who favor fire
But if it had to perish twice
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice
The narrator says that both fire and ice could destroy the world. The fact that he had a personal experience with both in a form of desire and hate. Fire and ice are similar in there own way.
The theme is hate and desire can cause destruction.
Fire is desire and ice is hate.