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It depends on you- if you think of the two are pasrt and parcel of the same thing, then use the singular. If you see them as separate then use the plural. I think both work. However, I would use the plural as they are not automatically linked IMO:
Corruption and decay are major themes...
In terms of grammar, either 'is' or 'are' is fine. However, within the genre of Literature, corruption and decadence are generally dealt with as separate themes, so using 'are major themes' fits nicely. On the other hand, if Eliot's poetry ties corruption to decadence, then 'is a major theme' would be the better fit.