As an individual with personal projects, probably nothing bad will ever happen even if you mangle licenses to their fullest extent.

The issue becomes when you then decide to build something commercial with photos/libraries/etc that have non-commercial/attribution features to them. If I wanted to use this theme to build a Startup, it could become very enticing to companies to target litigation against me. Above and beyond litigation, it could be a PR nightmare if press found out I had built something on top of code that I wasn't allowed to use. Think of all the questions it would raise -- well if he used non-commercial code commercially, what if he had stolen code from other places?

It also lends a sense of credibility to the project/photo/download in that the author thought about the way that his creation was to be used. We used a "free" open source iPhone mockup that seemed perfectly legitimate, and implemented it into one of our client's websites. A few months later, Getty Images had detected that a commercial copyrighted image had been used in the background of the iPhone mockup and sent a litigation letter to our client demanding payment. It was an expensive lesson for us to learn that just because something says it's one thing doesn't mean we can take them at their word.

"Free" for what? You need a license because you want to know exactly what you can and can't do, especially if you are a company. And it better be a well known license, that has stood in trial.

The worst thing that can happen is going to jail or having to repair whatever damage they claim you have caused. Though it's very unlikely in your case and for this library.

You can get sued.