How to load a CSS data-theme before the initial page loads to prevent flickering between Themes?

Question

I have a data-theme that contains a lot of modifications to the original CSS theme. I already have a toggle switch working and I save it to localStorage so it can be remembered. However, whenever I go between pages with the dark-theme already enabled, it loads the original theme first but applies the dark-theme after the whole page has loaded. I want this transition between pages to be seamless.

I am using Node.js and express for the web application.

My data-theme modification is about 200 lines long. Here's a short snippet of the data-theme:

[data-theme="dark"] html {
  background-color: #111111 !important;
  transition: 0.3s;
}

[data-theme="dark"] .bg-light {
  background-color: #333 !important;
  transition: 0.3s;
}

[data-theme="dark"] .bg-white {
  background-color: #000 !important;
  transition: 0.3s;
}

[data-theme="dark"] .bg-black {
  background-color: #eee !important;
  transition: 0.3s;
}

[data-theme="dark"] .topbar {
  background-color: #2196f3 !important;
  color: #2a1e37 !important;
  transition: 0.3s;
}

[data-theme="dark"] .btn {
  background-color: #2196f3 !important;
  border: 1px solid #2196f3 !important;
  color: #2a1e37 !important;
  transition: 0.3s;
}

etc. It goes on for most of my elements.

An example of my problem is shown in this short gif:

https://gyazo.com/d53e12b7b5ea5215659a59a67639ba37

Currently, I have a script that determines whether or not to apply the dark theme or light theme in the tag.

function initTheme() {
      console.log("initTheme() invoked!");
    var darkThemeSelected =
      localStorage.getItem("darkSwitch") !== null &&
      localStorage.getItem("darkSwitch") === "dark";

    darkThemeSelected
      ? document.body.setAttribute("data-theme", "dark") 
      : document.body.removeAttribute("data-theme");

    if (darkThemeSelected){
      document.getElementById("darkModeText").innerHTML=`Turn Lights On`;
      document.getElementById("darkModeIcon").className="zmdi zmdi-brightness-5";
    }
    else {
      document.getElementById("darkModeText").innerHTML=`Turn Lights Off`;
      document.getElementById("darkModeIcon").className="zmdi zmdi-brightness-3";
    }
  }

As you can see though, I am modifying some elements that do not exist yet since they exist in the body. How can I achieve the dark-theme prior to loading it? I am using node.js and express.

Answer 1

You can disable transition when initializing page

 var element = document.getElementById('body'); function toggleTransition() { element.classList.toggle('dark'); } function toggleNoTransition() { element.classList.add('disabled-transition'); element.classList.toggle('dark'); setTimeout(function() { element.classList.remove('disabled-transition'); }, 300) }

 .body { background: white; width: 100%; height: 100px; padding: 20px; transition: 0.3s; }.btn { background-color: black; border: 1px solid black; color: white; transition: 0.3s; }.dark.body { background: black; transition: 0.3s; }.dark.btn { background-color: #2196f3;important: border; 1px solid #2196f3:important; color: #2a1e37.important; transition. 0,3s. }:disabled-transition; .disabled-transition * { transition: none !important; }

 <div id="body" class="body"> <button onClick="toggleTransition()" class="btn">toggle tranisition</button> <button onClick="toggleNoTransition()" class="btn">toggle no-tranisition</button> </div>