The way it works is that your browser asks your operating system what theme you prefer:
no preference. The browser then exposes that value to any site you visit, allowing it to check what theme you prefer.
The auto detection relies on your OS having a light/dark theme switcher, and the browser being able to read that value from your OS.
If you choose dark for your OS, then we'll assume you'd like dark on the giffgaff forums, but you can always manually reverse that change or manually enable dark mode if you'd choose. I'm not planning to automate the OLED dark mode yet (as there isn't a way to check if a device has an OLED screen). You can still enable the OLED mode whenever you'd like.