Some say the world will end in fire
Some say in ice
From what I've tasted 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 authors tone is ironic because he is talking about fire or ice destroying the world. In this case fire is symbolizing desire and ice is symbolizing hate. It is ironic because the 2 things are complete opposites.
The theme is that fire being desire and ice being hate can cause destruction.
Fire represents desire and ice represents hate.