What is the theme of "To Autumn" by Keats?

In "To Autumn" Keats accepts the passing of this most radiant of seasons with calm and equanimity. However beautiful the luscious bounties of nature on display may be, he knows that they will soon pass, for winter is on its way. Yet at the same time, the imminent prospect of seasonal change does not upset him in the slightest. For he knows, as do we all, that autumn will come round once more when the time is ripe. And then, yet again, the poetic eye will behold a scene of "mists and mellow fruitfulness."
As with many of Keats's poems, the description of nature in "To Autumn" is both resolutely earthly, and yet at the same time filled with a transcendence that hints at another world entirely. It is the interaction between these two different, yet complementary worlds, that provides the poem's thematic basis.
It is the very transcendence of autumn that explains why Keats accepts its imminent transition to the harshness of winter with such remarkable composure. The turning of the seasons predates man's time on earth, and will continue long after Keats and everyone else has shuffled off this mortal coil. Keats revels in his lush, sensuous descriptions of the joys of nature. But he is not simply engaged in painting pretty word pictures. Keats's notion of beauty is eternal; it is unchanging and pure; and the fruits of nature, though prone to inevitable death and decay, partake of this higher beauty.
https://www.poetryfoundation.org/poems/44484/to-autumn

The theme of "To Autumn" is the transitory quality of nature.
In one of the marvelous letters written by Keats to friends, he explained that he composed "To Autumn" because

Somehow a stubble plain looks warm--in the same way that some pictures look warm--this struck me so much on my Sunday's walk, that I composed upon it.

The three stanzas of Keats's ode depict this tempered warmth of Autumn with its own beauty, although like the other seasons it is transitory.
In the first stanza Autumn has "conspired" with Summer, its "close-bosomed friend," a personification with suggests the mating process since fruit is then produced. Then, in the second stanza, the bounty of nature is harvested and Autumn sleeps after all her work, her

hair soft-lifted by the winnowing wind....Drowsed with the fume of poppies....

Finally, in the third stanza Keats underscores the importance of the role of Autumn and harvest time: "thou hast thy music too." For, there is a ground choir of gnats that "mourn among the river sallows," along with the robins, grasshopper, and crickets, who sing while the"twittering swallows" gather in the sky. These creatures express the melancholy in delight which Keats often felt. Autumn is the most bountiful of seasons, rich in its fruitfulness and the music of its sounds; however, winter does approach, signaled by the gathering swallows in the skies. Like all seasons and all life, it is temporal.