Port the css to css in js

[cuongcua90]

Hi material ui,

I create this issue to discuss about the way to port this repository styles to css in js. My team has used this repository and the css in js in my project also. I know, we have some discussion in the gitter chat about this problem. And the problem that @hai-cea concern is the way to make the theme configure. I think there are some straightforward way, but it is not much convenience now.

1. First, we can expose for example defineTheme() and reset the global style in material ui.
2. The other way, we can transform the styles props to the component and have mixin to process with the component styles.

Hope we have good solution for this.

[hai-cea]

Thanks @cuongcua90 - Let's move this discussion to #30