WebSchemeToggle/static/js/script.js

function load() {
  // setting the ID for the buttons used for toggling.
  // in order to support multiple buttons, e.g. to support collapsing navbars for mobile devices we create an array
  const buttons = document.querySelectorAll(".themebtn");

  // Get the stored value for the session
  function getStoredUseDark() {
    const storedUseDark = sessionStorage.getItem("useDark");
    if (storedUseDark != null){
      return (storedUseDark == "true");
    }
  }

  // Set the stored value for the session
  function setStoredUseDark(storeValue){
    const storeStringValue = ((storeValue == true) ? "true" : "false");
    sessionStorage.setItem("useDark",storeStringValue);
  }

  // MediaQueryList object
  const useDark = window.matchMedia("(prefers-color-scheme: dark)");

  // Toggles the "dark-mode" class based on if the media query matches
  function toggleDarkMode(state) {
    // Older browser don't support the second parameter in the
    // classList.toggle method so you'd need to handle this manually
    // if you need to support older browsers.
    document.documentElement.classList.toggle("dark-mode", state);
    setStoredUseDark(state);
  }

  // Initial setting of the preffered system setting
  function initialization(){
    const initUseDark = getStoredUseDark();
    if (initUseDark != true && initUseDark != false){
      toggleDarkMode(useDark.matches);
    }else{
      toggleDarkMode(initUseDark);
    }  
  }
  
  initialization();

  // Listen for changes in the OS settings
  useDark.addListener((evt) => toggleDarkMode(evt.matches));

  // Toggles the "dark-mode" class on click
  // We itterate over each button in the buttons array:
  buttons.forEach(button => {
    button.addEventListener("click", () => {
      const toggledStoredUseDark = (getStoredUseDark() ? false : true);
      toggleDarkMode(toggledStoredUseDark);
    });
  });
}

window.addEventListener("DOMContentLoaded", load);