site/docs/4.0.0-preview2/api/R/darkswitch.js

$(document).ready(function(){

  prefersDarkScheme = window.matchMedia("(prefers-color-scheme: dark)");

  currentTheme = localStorage.getItem("theme");
  if (currentTheme) {
    init_color_scheme_css("css", currentTheme);
    init_color_scheme_css("css-code", currentTheme);
  } else if (prefersDarkScheme) {
    if (typeof $mode === 'undefined') {
      $mode = 'light';
      if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) $mode = 'dark';
      init_color_scheme_css("css", $mode);
      init_color_scheme_css("css-code", $mode);
    }
    toggle_color_scheme_css("css", $mode);
    toggle_color_scheme_css("css-code", $mode);
  }

  // function to initialise the css
  function init_color_scheme_css($id, $mode) {
    if ($("#"+$id)) $("#"+$id).remove();  // remove existing id
    $("#"+$id+"-"+$mode).attr( {
      "data-href-light": $("#"+$id+"-light").attr("href"),  // store the light CSS url
      "data-href-dark": $("#"+$id+"-dark").attr("href"), // store the dark CSS url
      "data-color-scheme": $mode,  // store the mode, so that we don't re-initalise
      "media": "all",  // drop the media filter
      "id": $id  // rename the id (drop the `-{mode}` bit)
    } );
    $other = ($mode == 'dark') ? 'light' : 'dark';
    $("#"+$id+"-"+$other).remove();
  }

  // function to toggle the CSS
  function toggle_color_scheme_css($id, $mode) {
    // grab the new mode css href
    $href = $("#"+$id).data("href-"+$mode);  // use `.data()` here, leverage the cache
    // set the CSS to the mode preference.
    $("#"+$id).attr( {
      "href": $href,
      "data-color-scheme": $mode,
    });
  }

  // toggle button click code
  $("#css-toggle-btn").bind("click", function() {
    // get current mode
    // don't use `.data("color-scheme")`, it doesn't refresh
    $mode = $("#css").attr("data-color-scheme");
    // test if this is a first time click event, if so initialise the code
    if (typeof $mode === 'undefined') {
      // not defined yet - set pref. & ask the browser if alt. is active
      $mode = 'light';
      if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) $mode = 'dark';
      init_color_scheme_css("css", $mode);
      init_color_scheme_css("css-code", $mode);
      // `init_color_scheme_css()` any other CSS
    }
    // by here we have the current mode, so swap it
    $new_mode = ($mode == 'dark') ? 'light' : 'dark';
    toggle_color_scheme_css("css", $new_mode);
    toggle_color_scheme_css("css-code", $new_mode);
    // `toggle_color_scheme_css()` any other CSS

    // Saving user's preference to the localStorage
    localStorage.setItem("theme", $new_mode);
  });

});