William Shakespeare's Sonnet 55 explores the immortality of the subjects of poetry through the power of verse.  Shakespeare uses precise diction to paint a destructive image of time.  Phrases like "besmeared with sluttish time" suggest that time has the power to rob subjects of all recognition in the future.  Further, the mythological allusion to Mars the Greek god of war serves to strengthen the image of time as a brutal entity.  The poem, however, says that the subjects of poems (including this one) will live on in spite of time because the published word is everlasting as it is handed on to generation after generation.  Therefore, the theme of this sonnet is about the immortality of the subjects of poetry.