Evernote already has Dark Mode built-in, but now the app supports iOS 13's system-wide Dark theme toggle. What this means is if you set your iPhone to the Dark theme (dark mode everywhere!), then Evernote will follow suit. When you open the app, it'll have a black background.
The update for the cloud-based, note-taking app is now available from the App Store for the iPhone, iPad, and Apple Watch (if necessity really calls for it).
Source: iMore