@Kuba, yeah I just independently realized you might still have a prerelease installed. Re `StyleHints`, try something like `CurrentValue[$FrontEndSession, {StyleHints,
"GroupOpener"}] = "OutsideFrame"`. This is what the preferences dialog does (except on `$FrontEnd` rather than `$FrontEndSession`). I've been a bit loathe to advertise this because there are some inheritance issues that aren't 100% worked out, yet (but don't really cause problems at the current level of usage).