How does the monologue by Shylock in The Merchant of Venice that begins "Signor Antonio, many a time and oft..." relate to the themes of the play?
That quote sequence comes from Act 1, Scene 3. Earlier, Bassanio asked Antonio to lend him some money, so that he could go and woo Portia. Antonio can't lend him the money though, because all of his finances are currently tied up in a shipping business transaction. Antonio does agree to essentially co-sign for a loan from a different source though. That other source winds up being Shylock, the Jewish money lender in the play.
Shylock hates Antonio. I don't know which themes you are trying to focus on, but the dialogue from Shylock contains quite a few of the themes, hate being one of them. Shylock hates Antonio because Antonio is a Christian (which the quote doesn't indicate), because Antonio undercuts Shylock's business by lending interest-free loans, and because Antonio has publicly insulted and spat on Shylock.
Another theme of the play is greed. Shylock is a greedy character. I could argue that Antonio is too. The quote that you chose clearly shows that money is at the forefront of both Shylock's and Antonio's minds.
A third theme of the play is religious warring or persecution. Antonio and many other characters hate Shylock and abuse him, because he is Jewish. Shylock in turn hates them back because they are Christian.