WeeWilly
Android Enthusiast
When I first read about Marshmallow's System UI Tuner I thought it might be a feature similar to Nova Prime. Maybe someday, but for now the features are limited. To turn on the System UI Tuner long-press the Settings gear icon. After a couple of seconds you should see a message that says the System UI Tuner has been added to Settings (the option should appear at the bottom of the Settings menu, just below "About phone.") I haven't experimented with it yet, but here's an article that explains the various options.