Twenty Sixteen is designed by Takashi Irie, who also designed the Twenty Fourteen and Twenty Fifteen default themes. Irie describes Twenty Sixteen as:
A modernized approach of an ever-popular layout — a horizontal masthead and an optional right sidebar that works well with both blogs and websites. It has custom color options that allow you to make your own Twenty Sixteen. The theme was designed on a harmonious fluid grid with a mobile first approach. This means it looks great on any device.
The new theme has hints of Twenty Fourteen but is different enough to stand on its own as a unique design. It doesn’t look as modern as Twenty Fifteen out-of-the box but the design could change during the 4.4 development cycle.