UI.X for XenForo 1 was the original flagship product from ThemeHouse/Audentio. Though the look and feel was modernized in UI.X 2, we decided to use the same colors, fonts, and general look and feel as it was back in XenForo 1 for a dark version. UI.X Classic Dark has been updated with the UI.X 2 Framework, which comes with tons of new features. There may be some features it does not have perfect parity with, but many are indeed planned.
This is a free product with any purchase of UI.X 2 for XenForo 2 with an active license. Use coupon code ClassicDarkFreeUIX2 if you have UI.X 2 and ClassicDarkFreeUIX2Dark if you have UI.X 2 Dark. See UI.X 2 for coupons as well if coming from XenForo 1.

More UI.X Classic Dark Features:
Standard, floating, or sticky navigation and userbar
Right to left support
Dozens of different search bar and navigation position variations
Collapsible nodes, sidebar, postbit, and signatures
Many types of category description types (tooltip, inline, or none at all)
Social Media icons can easily be moved down to the footer or to the logo block
Different login styles such as modal window, dropdown, and slideout
Option to enable avatars based on Material Design's color system
Option to remove the forum list page actions (New Posts and Post New Thread buttons)