It would have been great if you would have done this in 3.5:
XmlnsDefinition("http://schemas.actiprosoftware.com/winfx/xaml/ribbon", "ActiproSoftware.Windows.Controls.Ribbon.Themes")
Rather than this:
XmlnsDefinition("http://schemas.actiprosoftware.com/winfx/xaml/themes", "ActiproSoftware.Windows.Controls.Ribbon.Themes")
Since you should avoid breaking changes in future revisions, don't change the latter, but please consider adding the former to the AssemblyInfo.cs. Since the namespace is in the same assembly, it makes sense (and is more convenient) to have the the same XML namespace.
XmlnsDefinition("http://schemas.actiprosoftware.com/winfx/xaml/ribbon", "ActiproSoftware.Windows.Controls.Ribbon.Themes")
Rather than this:
XmlnsDefinition("http://schemas.actiprosoftware.com/winfx/xaml/themes", "ActiproSoftware.Windows.Controls.Ribbon.Themes")
Since you should avoid breaking changes in future revisions, don't change the latter, but please consider adding the former to the AssemblyInfo.cs. Since the namespace is in the same assembly, it makes sense (and is more convenient) to have the the same XML namespace.