The central theme is :beauty is evident, or truthful in art. Both of these things last forever, or at least as long as the art in which they are embodied exists--for example, the urn or jar/vase on which these pictures are carved and painted. In the first few lines, the speaker is marveling at the pictures on the urn and asking questions as to the nature of the urn, the keeper of the secrets of the characters on its sides--who are they? What chase is going on? What music is playing? What love ecstasy is displayed? In the next few lines, the speaker talks of the music the piper is playing--because it is imagined, the music is sweeter than any we can actually hear since we each imagine music that we would like to listen to; the speaker also views the lovers who will always be young, always be excited with one another since they are always just on the verge of a kiss. The trees never lose their leaves, the musician will never tire of playing, the lovers will always be enticed with one another, and the speaker realizes that these characters exist as ideas and "above" the world of experience in which he lives which is less than perfect--"burning, parched". In the next few lines, the speaker notices other characters--the priest leading the cow to sacrifice, the quiet town, etc. The ending addresses the urn as "Cold Pastoral" who remains the same while we all grow old and die. The truth of the "ideal" life is beauty.
"Ode on a Grecian Urn" is quite confusing as far as it's theme is concerned. THere are diffrernt threds of theme.
2. Wisdom and knowlede
3. Beauty and art
4. Art and culture and life
All these themes can be well explained as the core of the poem at diffrent times with solid reference to the poem.
People say that beauty and art is the central theme as in the last stanza the speaker infers from the urn that beauty is truth and truth beauty. However, this statement of Keats has remained mysterious for ages.
It would be therefore a personal judgement as to which of the above is exactly the central theme.