Developer Steve Troughton Smith today tweeted photos of macOS 10.14 with some very juicy details about Apple's upcoming operating system. The OS is very clearly sporting a fresh new dark theme, presumably a toggle-able setting, with the dark UI affecting all application chrome. You can also see an icon for a Mac News app in the Dock, as well as a first look at Xcode 10.
Smith explains that the API the Mac App Store uses behind-the-scenes is including a video preview for Xcode, something that the current Mac App Store does not support. It represents a pretty big leak on Apple's part ahead of Monday's keynote.
Another major leak because Apple just uploaded a video to a place where everyone can find it. Good work, Apple.
As far as dark modes go - I'm generally not a fan, because they often feel like tacked-on afterthoughts, without designers really taking the implications into consideration. The only time where I saw "dark mode" work well was Windows Phone, because that UI was designed for it from the ground-up. Also, dark modes tend to be "dark", and not black. With today's modern displays with deep blacks, dark mode should really be black mode.