Change the color/theme of Office UI Fabric React components?

I'm trying to use Office UI Fabric React components in my web app. Is there a way to change the color or theme of the components? For example, I tried something like this:

ReactDOM.render(
  <DefaultButton
    className='my-button'
    text='Test Button'
  />,
  document.getElementById('root')
);

my-button is a css class defined as red background color. But actually it didn't change anything. The button background color is still the default #f4f4f4.

Is it possible at all to change the colors of the components?

[UPDATE] Ideally I think I want to globally change the theme of the components so my app can have a consistent look and feel.

Thanks!