Unium: Developing a minimalist puzzle game

A little while ago we started working on a simple puzzle game, with just about the simplest goal I can think of -- drawing a single line.  You're presented with a grid of tiles, and can start drawing a line from any point on the grid.  Any tiles you pass over with your line flip color, and your goal is to turn all the black tiles white with a single unbroken line.

There are a couple of wrinkles to the formula.  Grey tiles don't change color, so you can optionally pass over them.  You can also overlap the line, causing the underlying tile to flip each time: white becomes black becomes white again.

So far, this has turned out to be a pretty fun and contemplative puzzle game, so we're working our way towards making it into something we can actually ship.  Along the way we've developed a stark, fairly minimalist aesthetic feel that I'm pretty happy with.  Take a look and see what you think!