In Stanza 2, it says, They, too, aware of sun and air and water. "They" is referred to as the people from different countries. The sun, air, and water represent equality as we all need those 3 to live, relating to the theme of the poem.
In the sentence fed by peaceful harvests, by war’s long winter starv’d, the "war's long winter starved" refers to how we all farm during the time of peace, when there is no war. During war, a lot of people do not get food, and this happens to people all over the world. This again adds to the theme being discussed.
In the sentences Their hands are ours, and in their lines we read
A labour not different from our own.; the poet is explaining that every human has hands who all work equally. Everyone works and they are no different from each other. Again, this adds to the theme being discussed.