Realised needs to be changed from the past tense to the present tense so that it is realise. In this case, the verb is part of a prepositional phrase and is being used to talk about moment within a story set in the past.
Have forgotten needs to be changed to had forgotten since it is a past action that happened before another action in the past.
Bought and got need to be changed to buy and get so that the sentence is parallel. These words are part of a series of actions that starts with the wake.
I think the main theme that is best expressed in "The Gift of the Magi" is love. The reason why I think this is because Jim and Della were willing to make each other happy and they showed that the wealth of love makes up for the poor.