Tjhere are a number of possible themes we could point out in this famous poem, but the one that I feel is of particular relevance to today's society, given the environmental disaster that we have made and suffer, is the green message that is clearly present in this poem.
Let us remember that the Mariner is only able to pray again and expiate the curse that lies upon him when he realises fully the consequences of what was for him an unthinking and casual act. Killing the albatross was not done out of malicious intent, yet the Mariner has to accept responsibility for this crime. Part of this process is learning that all creatures in nature are just as special as other creatures. This is why when the Mariner is able to bless the water snakes, the albatross symbolically falls from his neck, as he has learnt this lesson and understands that all of nature must be cherished and possesses its own wonder and beauty.
This is why the final section of the poem in the third from last stanza seems to summarise this theme when it says:
He prayeth best, who loveth best
All things both great and small;
For the dear God who loveth us,
He made and loveth all.
As the Mariner has learnt, it is important for man to live in harmony with all of nature and all of its creatures. This is the only way to ensure that we can live in harmony with God.