Support dark theme

[omar10594]

Codepen with dark theme: https://codepen.io/omar10594/pen/VrWQRv

Material design guidelines:

At least the numbers on date picker should be white:

[dpoetzsch]

I will probably not have time this month for this. But I'd be happy to review a pull request

[dpoetzsch]

It is better now, since pull request #30, but still not perfect.

I just release the new version 1.1.1 that contains the fix :)