I chuckled a bit when I saw this bug. Not a huge deal, but a little jarring when it happens. It doesn't actually "turn off" dark mode, because if you go to another page it's back. I just inhibits the dark mode settings when the page reloads to show your posted reply. It also brings back dark mode if you edit a post while it's still in light mode.
https://boardgamearena.com/bug?id=230691
Edit: I just discovered that this also happens when you post a new forum topic (such as this one). How meta.
https://boardgamearena.com/bug?id=230691
Edit: I just discovered that this also happens when you post a new forum topic (such as this one). How meta.