The main theme is about the defiance against death. The poem encourages people to fight against the approaching death instead of accepting it passively.
Well, the central theme of 'Do Not Go Gentle into That Good Night' is the battle against death. Dylan Thomas is trying to tell us that death is a natural part of life, but we should not just go along with it without a fight. He talks about different kinds of people and how they might face death. For example, the wise men know that death is coming but they still regret that their words had no more impact. The good men see their deeds are not as great as they could be. The wild men who had a lot of fun in life still have a sense of loss when death approaches. And the grave men, even at the end, should not accept death easily. It's a very powerful poem about the human spirit's refusal to be subdued by death.