You can add a very basic dark mode for your website using this CSS snippet:
<!DOCTYPE html>
<html>
<head>
<style>
@media (prefers-color-scheme: dark) {
body {
background-color: #1B1B1B;
color: #E7E8EB;
}
a {
color: #3387CC;
}
}
</style>
</head>
<body>
...
</body>
</html>
It will apply automatically, when you choose the dark theme in your operating system.
Here is how to do that: