Toggle switch ionic data not saved in dark mod theme

hi i success fully create dark theme mod and its working in devices but after exit app toggle data not saved
need help how to save toggle data in app

hear is a my code

    <ion-icon slot="start" name="moon"></ion-icon>
      <ion-label>Toggle Dark Theme</ion-label>
     <ion-toggle slot="end"
import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';

  selector: 'app-folder',
  templateUrl: './',
  styleUrls: ['./'],
export class FolderPage implements OnInit {
  darkMode: boolean = true;
  public folder: string;

  constructor(private activatedRoute: ActivatedRoute) { }

  ngOnInit() {
    this.folder = this.activatedRoute.snapshot.paramMap.get('id');
  cambio() {
    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)');
    this.darkMode = !this.darkMode;
    document.body.classList.toggle( 'dark' );


i found in google search how to stored data but its difficult because i am new in ionic development and still learning any one help me to solve my problem

Hi, this is my solution for saving the selected theme.

To use it, just call ChangeTheme.initializeTheme (); in the initialize of your page.
Use ChangeTheme.setDarkTheme (true); to switch to dark mode.

import { OnInit, OnDestroy } from '@angular/core';
import { isBoolean } from 'util';

export class ChangeTheme {

    public static preferenceColor: string;

    constructor() { }

    static initializeTheme() {
        this.preferenceColor = localStorage.preferenceTheme;
        this.setDarkTheme (this.preferenceColor === 'dark');

    static setDarkTheme (darkColor) {
        const systemDark = window.matchMedia('(prefers-color-scheme: dark)');

        if (darkColor) {
            this.preferenceColor = 'dark';
        } else {
            this.preferenceColor = 'light';

        document.body.setAttribute('data-theme', this.preferenceColor);
        localStorage.preferenceTheme = this.preferenceColor;

    private static colorTest(systemInitiatedDark) {
        if (systemInitiatedDark.matches) {
            console.log('test data-theme', 'dark');
            document.body.setAttribute('data-theme', 'dark');
        } else {
            console.log('test data-theme', 'light');
            document.body.setAttribute('data-theme', 'light');