That's only the beginning. Definitions are used to establish the playing field for both parties to play on. In our case throwing dictionary meaning show that the opponent isn't using word correctly, as well as providing a somewhat credible source for your own definition.
As for themes, you don't need one for rebuttals (which I mostly used if I think I know what we're talking about). You just need someone with a flawed argument, or an argument with holes.
To give an analogy, you can take down an opponent's castle by poking at weak points or attacking a shaky foundation. You don't need to do it in any particular way/order, nor do you have to counter by building a bigger castle of your own.
That is, you don't have to prove you're right. You just have to show they're wrong - eg. by giving a dictionary definition.