What is the theme of "Poem in October," and what is the tone of it?

"Poem in October" by Dylan Thomas is a poem written on the occasion of the speaker's 30th birthday. This is an interesting poem because poems about autumn, especially nature in that season, often focus on the transition from summer to fall (as marked by oncoming death and decay). In Thomas's poem, the tone is calm and pleasant as the speaker observes and describes the beauty of the natural world he observes on the morning of his birthday.
About halfway through the poem, the speaker reflects on birthday memories from his childhood and the tone becomes quite joyful. The speaker even includes imagery explicitly related to summertime to describe his mood on this birthday. It seems like a possible theme could be that birthdays give us a chance to reflect on our place in the world around us and to recall with gratitude the milestones of our lives.