If you’re pressed for space and clarity, using black for general interaction works well because it integrates in the system bars and lets you focus on the content, especially when the colors are defined by the user. Also, focusing on the user’s physical abilities and mindset is paramount for a good experience.

One thing we underestimated, despite intense research and good progress on the presentation side, was the hardcore user’s need to customize their calendar experience down to the tiniest detail (for example, changing the font’s transparency). Only after this initial UI exploration did we realize how much work would be needed — not only to implement the design suggestions, but also to customize everything.