Via Lukas Mathis:

Applications are often said to be inconsistent with their host system if they look different from other applications on that system. However, look alone doesn’t really matter. As long as users are capable of easily identifying the different UI elements, applications don’t have to look like their host system.

People see all kinds of different user interfaces while surfing the web. A different look doesn’t harm usability, as long as the individual widgets are easily recognizable, and as long as they behave the way the user expects them to.