In macOS Mojave in Dark Mode, dark colors are used in place of light colors in the user interface. For example, in Dark Mode the menu bar at the top of the Mac UI is no longer white with black text. Instead, it’s inverted so the bar is a dark color and the the text is white. That dark theme is carried throughout the UI.
For some, Dark Mode may not seem like a big deal, but Dark Mode has many benefits. The higher contrast of text makes it easier to read documents and menu items. Using dark colors instead of white reduces glare, which in turn reduces eye fatigue. Most importantly, Dark Mode can improve your ability certain aspects of video and photos, such as color and detail.