I am glad you got it sorted out and thanks for coming back and posting the solution.
Synchronized theme, BTW, is for folks with multiple Windows devices (tablets, Windows phone, PCs, notebooks, etc.) who want all their devices to look the same. It works when you have Sync settings enabled on all the devices and you sign in with a Microsoft Account instead of a Local account. If you have Sync enabled on other devices and use a MS account, that might explain it. If you don't have other devices with Sync enabled, then maybe (I am guessing now) Windows just picked something at random.