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
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