index.js

let ThemeComponent = React.createClass({
  getInitialState() {
    return {
      'theme': 'Day',
    };
  },
  getStyle(element) {
    return [baseStyles[element], themeStyles[this.state.theme][element]];
  },
  render() {
    return (
      <View style={this.getStyle('container')}>
        <Text>Hello!</Text>
      </View>
    );
  }
});

const baseStyles = StyleSheet.create({
  container: {
    flex: 1,
  },
});

const themeStyles = {
  'Day': StyleSheet.create({
    container: {
      backgroundColor: '#FFFFFF',
    },
  }),
  'Sunset': StyleSheet.create({
    container: {
      backgroundColor: '#EFEEE4',
    },
  }),
  'Night': StyleSheet.create({
    container: {
      backgroundColor: '#353739',
    },
  }),
};