UpdatedReducer.js

const initState = {
  theme: 'default'
}

const Reducer = (state=initState,action) => {
  switch (action.type){
    
    case "CHANGE_THEME":
      return {
        ...state,
        theme: action.theme
      }
      
    // you can have as many case statements as you need
      
    default: 
      return state
  }
}

export default Reducer