FYI, the XenForo dark/light style actually respects your system and browser settings. So you have three options:
1. pick the light style (sun icon at the top or Lapis Light at the bottom).
2. pick the dark style (moon icon at the top or Lapis Dark at the bottom)
3. pick the "default style" at the bottom. This uses your browser setting for Dark/Light:
There are apps in the windows store that change the current mode based on the current time, so it would change to dark after sunset and to light after sunrise.
Definitely dark, though I used light for the longest time. But I use dark themes everywhere these days - not least because I sit in a very darkened room and my eyes seem to have grown sensitive to the light in recent years.