“Classic Header” is now “Header Options”

And it allows you to un-stickify the header and make it’s width fixed (which might be useful if you keep trying to click the blank space where the Tumblr logo used to be, like me.)

It also allows you to make the notification bubble on the Inbox red again, so you wouldn’t miss new messages.

If you already have “Classic Header”, you can perform an Update All from XKit Control Panel > Other > Update All to upgrade.