What I am about to share with you is an ancient secret. And despite @TreePreserverMan's negativity, I will share it with you anyway so others can enjoy. Forums dark mode DOES exist, It's just not built into XenForo. You can use dark mode on the forums with the extension stylish. All you have to do is install the style created by @TheDiamondPicks The original thread and installation instructions created by @TheDiamondPicks can be found here. You're welcome in advance. I take no credit for this plugin.
Yeah it still works: http://itsjerryandharry.com/threads/j-h-forums-android-app-pre-release.107666/ But it's not particularly useful, it's just a thread reader really.
criticism is now 'negativity'. whoa whoa, nobody disagree with an utterly useless idea! how dare they infringe upon my mental bubble..