It’s the default (as in default selected theme) for desktop. I’m not sure if that’s the case for Android, I unfortunately don’t own an Android phone currently to check.
The prefers-color-scheme being reported as light seems to be coming from Firefox’s resist fingerprinting feature (which is always enabled on Tor browser).
This website always reports light for me as well, same goes for manually checking with JavaScript. I think that value gets spoofed regardless of the enabled theme.
Relevant issues:
Edit: added another relevant issue
For me it’s dark mode by default. 🤔 I use MacOS but I think it should be the same on all operating systems.
I addressed how this affects fingerprinting in this comment.
The short answer is probably not.
The longer answer is that in theory it may be possible to use some kind of exploit or side-channel attack to figure out what theme is enabled or which themes are available through media queries for example with JavaScript or CSS. Tor browser uses dark mode by default, and since this is a dark theme, that doesn’t change. It only affects the chrome (UI) of the browser, not the page itself.
So that is something you should keep in mind for your threat model.
My answer is based on personal experience, some basic googling and asking the Tor community. But it still can’t hurt to do some additional research yourself. 😄
Feel free to correct me if I’m wrong.
You should open an issue for this if it doesn’t exist yet, or, if possible, attempt a pull request. The maintainers are kind, welcoming and helpful from my experience.