How To Prevent Theme Colour From Flickering in Svelte Or SvelteKit

Ever since I started using Svelte, my theme colours has always been flickering on page load. Finally I found a workaround

Problem

At first, I was changing my website's theme colour inside the onMount function but the theme colours were always flickering. The reason was that, the body loads before the onMount function sets the theme colour.
Here's a gif showing the problem

So as you can see, when the page is reloaded, the light theme is first shown, then after the document has finished loading, the onMount will then set the theme colour

Solution

The only probable solution is to set the theme colour before the body has even loaded. How do we do that, we insert a script tag inside the head element and this code will run before the body is loaded. That's the best way we can prevent the flickering of colours.

The Code

Okay, so you can write this code in the component in which you use to toggle in-between the themes.

<svelte:head>  <script>    if (document){      let mode = localStorage.theme || "light";      if (mode === 'dark' || (window.matchMedia('(prefers-color-scheme: dark)').matches)){        document.documentElement.classList.add("dark");        localStorage.theme = "dark";      }      else {        document.documentElement.classList.remove("dark");        localStorage.theme = "light";      }    }  </script></svelte:head>

Explanation

In order to access the head element, we used the <svelte:head> component. Then we created the script tag just as we would on our normal HTML pages. The next statements are the important ones, the reason why we used if (document) is that, this code first gets evaluated on the server before being rendered on the client, so if you try to access document on the server, it will pop up an error.
I know SvelteKit provides the { browser } constant through the $app/env module but mind you, this is not available in the custom script tag we made, you will need to use your own workaround and that's why we are using document to check.
Then on the next line, we try to retrieve the theme from the localStorage, if it's not set, it defaults to "light" theme;
Then the next steps are the addition of classes and setting of the theme in the localStorage.

Now look at how the page loads without flickering

Happy Coding!