a11yist/app/javascript/controllers/set_theme_controller.js

import { Controller } from "@hotwired/stimulus"
import Cookie from "../lib/cookies"

// Connects to data-controller="set-theme"
export default class extends Controller {
  COOKIE_NAME = "modeTheme";

  connect() {
    const cookieValue = Cookie.get(this.COOKIE_NAME);
    if (cookieValue) {
      return true;
    }
    const darkMode = window.matchMedia("(prefers-color-scheme:dark)").matches ? "dark" : "light";
    Cookie.set(this.COOKIE_NAME, darkMode);
    window.document.getElementsByTagName("html")[0].setAttribute("data-bs-theme", darkMode)
  }
}