message: Picker modal Back Button Style
  headerBackButtonTextStyle prop option added for picker.
diff: @@ -144,7 +144,7 @@ class PickerNB extends Component {
  this._setModalVisible(false);
  }}
  >
  - <Text>{this.props.headerBackButtonText || "Back"}</Text>
  + <Text style={this.props.headerBackButtonTextStyle}>{this.props.headerBackButtonText || "Back"}</Text>
  </Button>
  </Left>
  <Body>
  