IMPORTANT NOTE: It has been reported that Night Owl has since changed ownership and now includes malware-like behavior, contacting Internet servers and possibly using your Mac as a market research tool. Its own terms of service seems to confirm this. I will leave this video here, for now, for historical reference but do not recommend installing this app on your Mac any longer.
Dark Mode in macOS Mojave is a great new feature, but Apple didn't give us a way to quickly switch between light and dark modes. The little donation-ware app NightOwl gives you that control in your menu bar. You can also set a keyboard shortcut, select apps to stay in light mode, and set specific times to switch between modes.