Light/dark mode: the lazy way

If you already have a website working, and you haven't used any styles (no custom background, no custom color, no custom inputs), then you can use:

<meta name="color-scheme" content="light dark" />

This will enable both default light and dark modes for your website and will follow your system mode (no toggle available in the UI).

You can read more about it here: https://web.dev/color-scheme/

Note: this will work on both Chrome and Safari, but not Firefox