The dark mode feature uses local storage to store a user's preference for future usage. The problem is that when the dark mode is enabled and the page is reloaded, there's a flicker of white background all over the page before it turns dark. This happens for a fraction of a second and doesn't look natural.
Following is the implementation we normally do when setting up this feature. This works by detecting if the dark mode is enabled from localStorage and adding a CSS class called dark to the <html> tag so all elements that are descendants of that element will turn to dark styling.
<html>
<head>
...
<link href="./light.css" />
<link href="./dark.css" />
</head>
<body>
...
<script src="./main.js" />
</body>
</html>
// main.js
//...
if (window.localStorage.getItem('theme') === 'dark') {
document.documentElement.classList.add('dark')
}
However, the dark styling is only applied if the main.js file is called so it will be expected to see the flash of light styling.
To fix this, put the scripts inside the <head> tag, even before the <link> or <style> tags:
<html>
<head>
<script type="text/javascript">
if (window.localStorage.getItem('theme') === 'dark') {
document.documentElement.classList.add('dark')
}
</script>
...
<link href="./light.css" />
<link href="./dark.css" />
</head>
<body>
...
<script src="./main.js" />
</body>
</html>
By doing this, the page rendering will be blocked when the engine detects the <script> inside the <head> tag. While the renderer is idle, the JavaScript interpreter will assign the dark value to the CSS class list of <html> before the dark.css is loaded.