It appears the default is light mode, rather than system preference. For those of us who use websites logged out most of the time, this is a bit annoying. Hopefully the default is changed to system preference in future.
Also, I don't want to use Github when authorized to do things on company repos when browsing something completely unrelated in my own time and own devices. Thus this option is not available to me without creating a dummy account. I also have to change this manually whenever environment changes. With the accompanying blinding flash of light on unfortunate late night sessions.
I'll admit there might be other considerations than funnelling users to registering and using their accounts. It's just not a good user experience and misunderstanding of the target audience.