The theme of "Sonnet 18," although open to interpretation, is eternal beauty through verse.  This is known as possibly "the" best of Shakespeare's sonnets beginning with the immortal line, "Shall I compare thee to a summer's day?"  It is one of the many sonnets written to the elusive "young man," that no one has ever been able to pin down.  In regards to the theme, at the beginning of the poem, the young man is said to be "more lovely and more temperate" than a day in the season of summer.  The speaker then immediately gives many examples of summer day issues:  wind, length, heat, etc.  By the end of the poem, it is clear that the reason why "thy eternal summer shall not fade" is precisely because Shakespeare has written this poem.  Published poetic verse truly does lend immortality to the subject it's written about.  Therefore, the theme of "Sonnet 18" is summed up quite nicely in the last two lines:

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