It’s dead simple to update plugins within the Community Plugins Settings screen. To update a theme one must go to Appearance → Themes → Scroll to find their active theme → Hit update.
Show an Update Theme button below the Manage button in the Appearance Screen.
Optional: show what theme is currently installed.
Bonus Points: Update all themes
It might even be good if theme updates were based on a similiar concept to plugins, using a manifest.json file in the repository which provides a few additional details, most importantly version number.
BRAT can update themes automatically. But its basically doing a file date check. It would be better to have a “release” package for a theme, this way theme devs can check in changes during development, but without release the updates officially to the public until they are finalized.