I have added a bootstrap-like theme.
It's not totally like bootstrap but it's easily customizable, I've done the CSS with SASS so you can quickly change the color and the style of the selected buttons. See ".selected-button" class and _mixins_and_vars.scss file
There are also minor changes in Picker.Date.js :
Here are some captures, hope you'll like it!
Changed months and years to be in tables instead of divs
Reduced css for fitting with tables
Thanks, nice theme.