The xamRibbon is designed to emulate the look of the Microsoft Ribbon UI. As such it's look isn't related to the OS theme so many of the pre-registered themes relating to OS themes (e.g. Aero, LunaNormal, LunaOlive LunaSilver, etc.) will not cause the xamRibbon to appear different.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2