"Compact" is the built-in compact styling. Right-click (or otherwise context-click) on the toolbar, select "Customize.." and near the bottom there is a "Density" selector that you can set to "Compact". Note that this is somewhat independent of the color scheme used.
I'm pretty sure that the "Compact" layout is available in beta 57 as well, not just developer edition. It's not in release 56, though; it was added for 57.
