JS & ES6: Access static fields from within class

You can access it as you would expect, however if I recall there were some issues when using Babel and exporting the class immediately, so export after defining the class if you're having problems:

class MyStyle extends Stylesheet {
   static Color = {
      mainDark: '#000'
   }

  someMethod() {
    console.log(MyStyle.Color.mainDark);
  }
}

export default MyStyle;

You can read more about the Babel issue in an answer Marian made on a similar question, which is supposedly fixed in Babel 6.2.1.