Android implements what OEMs and ROMs have for ages. The Developer Option has an almost nondescript "Show SystemUI Tuner" that, once enabled, adds a SystemUI Tuner setting to the main Settings list.

If you noticed the SystemUI Tuner missing from the new preview 3 of Android 6.0 Marshmallow, this is how you turn it back on.

One of the standout features of the new version of Android O is the ability to customize the nav bar, but the feature exists in all Android Nougat versions.

As is typical, Google already stripped out all the other System UI Tuner features introduced in the earlier previews with DP3.