Google updated its design spec recently. The material spec, which Google says is a living document (as evidenced by its ongoing updates), gained further guidance on floating action buttons, dialogs, updates on typography, and a lot more.
One less-advertised update was a change to the section in “Structure” concerning the navigation bar in Android. The “color variants” text is still identical to that from the “status bar” subsection, but an image showing nav bars themed to match your device’s hardware was removed.
Android’s System Bars And Your Interface – Whether And How To Theme was written by the awesome team at Android Police.