Most devices have a dark mode for the screen these days. Up to now the forum has ignored your system preference and been permanently in light mode, but it’ll now follow whatever you set on your phone or computer.
If you want to keep light mode for the forum specifically but have the rest of your device in ‘dark’, you can adjust this in your user preferences under ‘Interface’. Change the ‘Dark Mode’ option from ‘Dark’ to ‘Same as regular’.