Ah, yes, as Ernest says, this is a very good piece of advice I received early on too - create a new theme, using the theme you purchased, rename it with something like "child" at the end, and use that as your default, and the target of all your edits.
Xenforo keeps things solid by not bloating their software. People develop addons because they want certain functions and features. For newbies, it could get overwhelming trying to take in all Xenforo has to offer, what you get with the default theme, and what you'll need as far as addons. My advice - ask questions, we all start somewhere and it's certainly tougher on your own.