Transparent default skin

## Current version

The current default skin is grey-ish.

![digit-current](https://f.cloud.github.com/assets/378023/1401803/7a613848-3cdc-11e3-9c9c-a4eeb0153887.png)

It works ok, for prototyping/wireframing, but is pretty limited for production use and will most likely need to be customized. So instead of having to override all the colors for all components, here a somewhat easier way.

## New version

The idea is to keep the background of all components transparent so that the background underneath can show through. In addition all components inherit the color for border, text, thumb element etc. So in a blank un-styled app it would be black on white:

![digit](https://f.cloud.github.com/assets/378023/1309361/3876bb96-31de-11e3-9bba-171cdefa994d.png)

But we can change the `color` and all components inherit it:

![digit-color](https://f.cloud.github.com/assets/378023/1309397/d3434c34-31de-11e3-93c2-b8d3847e2207.gif)


### Some more benefits:

- You can change the color on a global level, or limit to different sections.. or just a single component.
- You can change colors at runtime. Also animated when switching.
- It looks similar to [iOS 7](http://www.sketchgems.com/wp-content/uploads/2013/06/iOS-7-UI-Kit-Lite.png) and the flat/colorful style that currently everyone is using.
- The light and dark skin are unaffected and can be used like before.

### Concerns:

- When you set the color to very light, it doesn't really work since some components like the toggle, radio, checkbox use white for the icons/thumb. But it's not too hard to customize it if you want to use white as color.
- I haven't tested performance when using transparent backgrounds. But just as a guess: Since most components use `border-radius`, painting will be slower anyways. If you're really concerned about the fasted performance.. then all components would need to be opaque and a rectangle. So maybe not a too big concern.