The forum’s color scheme will automatically switch light and dark color schemes if your system supports it. You can override it in your account’s interface preferences.
Here’s an example from macOS. The OS has three color modes which you can see in the system preferences box positioned on top of the forums:
- Light which means always use the light color scheme
- Dark which means always use the dark color scheme
- Auto which (usually) uses light during the day and dark at night (you can usually set the schedule)