Enable customized style on SketchField component
After customizing the color and other styles of the SketchPicker component, it is often necessary to alter the style of its sub component SketchField. In cases where dark background is applied on SketchPicker, the default black font for HEX/RGB beneath the input field won't show up.  Currently I need to target the color property defined inline like in issue #596 . Would be great if there is a more direct way to alter the style.

```
  .sketch-picker {
    span {
      color: white !important
    }
  }
```