In a way, it's hard to be clear about the theme of "Ode to a Grecian Urn" because two of the key lines are still debated. The debate is about the meaning of:
'Beauty is truth, truth beauty'--that is all
Ye know on earth, and all ye need to know.
The debate also encompasses a question about to whom the final lines of the ode are addressed. Some opinions are that they address the urn, the reader, or the figures on the urn. An ode is a lyric poem in high diction written to a person or thing, usually absent. This is a five stanza ode that does not adhere to a strict three stanza strophe, antistrophe, epode form. It has a unique rhyme scheme that varies per stanza. For example, using different rhyming words, stanza one has this rhyme scheme ababcdedce, while the second stanza has this one ababcdeced.
A defensible analysis is that, while the final lines of stanza four address the "little town" pictured on the urn, the fifth stanza addresses the Grecian urn just as the first does. The fifth stanza starts with the apostrophe "O Attic shape! Fair attitude!" "Attic" mean of Grecian origin. "Fair attitude" is a metonymy based on the urn's graceful form. If this is taken to be correct, then the debated lines about 'Beauty is truth, truth beauty' can be analyzed as the main theme of the ode.
In the fourth stanza Keats asks a good many questions, e.g., "Who are these coming to the sacrifice?". The ending lines on beauty answer these questions saying that the urn represents beauty and that is all the viewer needs to know about the urn. In other words, the urn should be enjoyed and valued for the beauty it offers whether the story behind it can ever be sorted out or not. Thus the main theme would be that beauty is all the truth that need be known about art and that the truth about the importance of art is that it gives beauty.
the theme of the poem "the grecian urn" is that art is eternal. art lives forever. the poet says that the urn which is found has the story of that era. the story of that village. we donot know that village and that time now because that is gone long ago. but we do know it through art and artist who has painted that urn. that urn is very precious because it tells us the story of that age which is passed. all the charecters who are painted on the urn are eternal because they will live thare till the end of earth. the generations will pass, one generation comes then the next and so on but the urn will remain there to tell the story of its time. and so the beauty will remain the same and everybody will enjoy the art in the same way as the poet himself.