[React Native] DarkMode, themes

# DarkMode

1. useColorScheme()
   • react-native API
     ▶ Refer to https://reactnative.dev/docs/usecolorscheme
2. Themes
   • react-navigation
     ▶ Refer to https://reactnavigation.org/docs/themes/
   • NavigationContainer 에 적용
   • 모든 스크린에 적용됨

import { useColorScheme } from 'react-native';
import {
  NavigationContainer,
  DefaultTheme,
  DarkTheme,
} from '@react-navigation/native';

export default () => {
  const scheme = useColorScheme();

  return (
    <NavigationContainer theme={scheme === 'dark' ? DarkTheme : DefaultTheme}>
      {/* content */}
    </NavigationContainer>
  );
};