The CSS skin Alex cited is called Flat UI [1]. FlatSpark is basically a port from that skin, our main goal was to be as close to that as possible. That's why FlatSpark has some limitations regarding customization, we just tried to port the skin, maintaining its colors and pre-defined sizes.
Creating the skin for FlexJS based on the CSS version, in this case, would be definitely easier. [1] http://designmodo.github.io/Flat-UI/
