how was this novel an allegory and what was one theme from siddhartha

Siddhartha can be seen as an allegory in many different ways. The story does touch on the Buddha. I think a greater idea could be the struggle to understand his natural passions (desire/samsara)to a life of asceticism.

Oh, there are again many themes to pick up but I'm going to go with the obvious, "the search for spiritual enlightenment".