With me, it's similar with running speed tests at night, such as while travelling with someone in a car or on a bus. Even with my phone screen dimmed to minimum, a white page background still gives off plenty of light. This is especailly noticeable when I switch back and forth between the signal meter app (Network Cell Info) which has a black theme and then back to TestMy on the web browser. This is where I like the dark theme.
On my PC, I'm happy with the regular white theme particularly during the day time. So with me, it's difficult to say which theme I prefer most as it depends on the situation, a bit like giving my preference on whether I prefer the room light on or off. ?
Edit: One issue I've run into with the dark mode is the Express Test does not show the progress meter and the percentage stays at '0%' until the test completes:
It does this in both Firefox, Chrome and on my mobile Firefox browser also. The does complete successfully despite showing no progress. The regular and fixed block size tests show the progress fine in the dark mode.