index.js

/* eslint-env browser */
import "./style.scss";

let theme = "light";
const themes = {};

themes[theme] = document.querySelector("#theme");

async function loadTheme(newTheme) {
  // eslint-disable-next-line no-console
  console.log(`CHANGE THEME - ${newTheme}`);

  const themeElement = document.querySelector("#theme");

  if (themeElement) {
    themeElement.remove();
  }

  if (themes[newTheme]) {
    // eslint-disable-next-line no-console
    console.log(`THEME ALREADY LOADED - ${newTheme}`);

    document.head.appendChild(themes[newTheme]);

    return;
  }

  if (newTheme === "dark") {
    // eslint-disable-next-line no-console
    console.log(`LOADING THEME - ${newTheme}`);

    // eslint-disable-next-line import/no-unresolved
    import(/* webpackChunkName: "dark" */ "./style.scss?dark").then(() => {
      themes[newTheme] = document.querySelector("#theme");

      // eslint-disable-next-line no-console
      console.log(`LOADED - ${newTheme}`);
    });
  }
}

document.onclick = () => {
  if (theme === "light") {
    theme = "dark";
  } else {
    theme = "light";
  }

  loadTheme(theme);
};