What is the theme of poem The Cloud by Shelley?

The poem's theme is a celebration of creativity. It praises the spiritual and physical powers of the cloud, and in doing so, shows the creative aspects of nature, which are reproduced by the artist as he creates his work.
On the physical side, the cloud speaks to us about how it bring rain for the flowers, as well as shade. It also provides the beautiful mists that cover the sun, the moon, and the mountaintops, and contributes to the rainbow. The artist also brings beauty.
Phrases such “as still as a brooding dove” allude to the spiritual side of the cloud. This phrase is borrowed from Milton's Paradise Lost, in which the holy spirit is described as “Dove-like brooding on the vast Abyss.”
The cloud, which dances and plays, acts as a creative spirit in this poem. The cloud is constantly transforming itself and enjoying everything it does, while fertilizing and beautifying the earth. Art does the same—and all of this is to be celebrated and enjoyed.