theme togller

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body class="modByTheme">
    <header>My page</header>
    <button class="themeToggler"></button>
    <p class="modByTheme">This is a paragraph</p>
    <b class="modByTheme">This is bold text</b>
    <br />
    <i class="modByTheme">italic text</i>
    <div class="modByTheme" id="last"></div>
  </body>
  <style>
    .light-theme {
      color: rgb(4, 33, 202);
      background-color: seashell;
    }

    .dark-theme {
      color: white;
      background-color: rgb(11, 20, 20);
    }

    header {
      padding: 3.5%;
      background-color: purple;
      text-align: center;
      text-decoration: underline;
      font-size: x-large;
    }

    #last {
      padding: 30%;
    }
  </style>
  <script>
    "use strict";
    window.onload = () => {
      console.log("asd");
      if (typeof Storage == undefined) {
        document.body.innerHTML = "Your browser is to weak!";
        return;
      }
      if (localStorage.getItem("theme") == null) {
        localStorage.setItem("theme", "light");
      }
      applyTheme();
    };

    const getOpTheme = (theme) => {
      return theme === "dark" ? "light" : "dark";
    };

    const applyTheme = () => {
      const theme = localStorage.getItem("theme");
      themeToggler.innerHTML = getOpTheme(theme);
      const modByThemeElems = document.querySelectorAll(".modByTheme");
      //console.log(theme);
      for (let elem of modByThemeElems) {
        if (elem.classList.length == 2) {
          elem.classList.remove(`${getOpTheme(theme)}-theme`);
        }
        elem.classList.add(`${theme}-theme`);
      } //*/
    };

    const themeToggler = document.querySelector(".themeToggler");
    themeToggler.addEventListener("click", (event) => {
      event.preventDefault();
      let newTheme = getOpTheme(localStorage.getItem("theme"));
      localStorage.setItem("theme", newTheme);
      applyTheme();
    });
  </script>
</html>