What Is The Theme Of Shakespear's "Sonnet 18"?

In sonnet 18 shakespeare praises the beauty of his beloved and begins by asking a question:

"Shall i compare thee to a summer's day?"

In the very next line he answers his question by saying that the beloved is far more lovely and consistently beautiful, since summer is short-lived and the summer sun is unreliable. Above all he thinks his beloved's beauty has an edge over that of a summer's day because it is intransient. He says that neither time nor death can spoil the beauty of his beloved. One may well question this as beauty also declines with time and growing age but in the last two lines he explains the reason for it:

"So long as men can live, or eyes can see
So long lives this and this gives life to thee".

'This' here refers to this poem of course. So the theme of this poem can be understood to be the intransience and immortality of art. It has been contrasted with human beauty and even the forces of nature which forever keep on changing and are thus inconsistent and transitory, but his poetry is not. Shakespeare's prophecy that his art will live on forever has proved to be true since he's regarded as the most celebrated poet and dramatist of English Literature and his works are are still widely read all over the world, although it's been 5 centuries since he wrote them.