Answer:
Since you are looking for multiple themes, your answers are A. love of nature and B. the cycle of life. Those are two central themes of the poem; they are intertwined and can't function without each other.
Explanation:
The poem is about the wonders of nature and its many relations to the human life itself. The rainbow is a metaphor for nature's beauty, and the speaker's adoration, joy and childlike curiosity signify the bond he feels with nature. If it weren't for this bond, he wouldn't feel alive at all.
This very bond with nature is the source of a person's continuity. "The Child is father of the Man" - but only if the man doesn't forfeit his connection with the nature as his source.
This poem makes no mention or allusion to God, nor the old age's wisdom. On the contrary, an old man can lose his wisdom if he were to cut off his relations with nature.
