How does the form of Shakespeare's Sonnet 18 "Shall I Compare Thee to a summer's day" establish the theme?What is the theme?

I'm not sure what you mean by the term "form", but if what you mean is the content and how it implies the theme, then the first line, "Shall I compare thee to a summer's day" establishes the nature theme that prevails throughout the sonnet. This is because each line draws a comparison between the receiver of the poem and an element of nature. Take for example the next line, "rough winds do shake the darling buds of may". This line evidences the theme of nature established in the first line because it is again a comparison between the rough winds (an element of nature) to the receiver of the poem.