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:
Firefox:
Windows

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.