I'm a big fan of Root Explorer for its powerful featureset and no-nonsense user interface. As utilitarian as it is, no one has ever accused it of being, well, pretty. But the latest update to the app, version 4.0 on the Play Store, gives the entire thing a fresh coat of paint. Yeah, it's somewhat overdue, but dedicated users will be happy to see it anyway. "Material Design" is the only entry in the changelog, but to be fair, that's a pretty big change.
Above: old and busted. Below: new hotness.
The switch from the Holo interface (which, if you'll recall, got its start with Ice Cream Sandwich) to Material Design doesn't make the app altogether revolutionary, but readability and layout are notably improved. The developer included both a dark and light theme, and the handy "sort order" and "view mode" commands can still be found in the main drop-down menu instead of the larger Preferences menu.
The updated version of Root Explorer is available in the Play Store now, and it doesn't seem to have any sort of rollout delay. I've found that Root Explorer (and a few other file browsers) aren't working on the Android N developer preview - presumably there are some bugs to work out with the new OS version.