A new feature of macOS Mojave is Dark Mode. This favors light text on a dark background instead of dark text on a light background in the system and standard apps. You can see the change in the menu bar, app toolbars and sidebars, and even the backgrounds of apps like Contacts, Calendar, Reminders, Notes, Mail and Maps. Not all apps will change, especially third-party apps. Document content and images will not change either, as they keep using the true color specified in the document. You can also change the accent color used in standard interface elements throughout macOS.