How to reproduce: create a DPI-aware application and set the AllowAero property of the ribbon form to false, the titlebar will not be themed

Workaround: override the ScaleControl method of in the RadRibbonForm instance in the project
Protected Overrides Sub ScaleControl(ByVal factor As SizeF, ByVal specified As BoundsSpecified)
    MyBase.ScaleControl(factor, specified)

    If Me.HasOwnToolbar AndAlso Me.AllowTheming AndAlso Not Me.IsDesignMode AndAlso Me.IsInitialized Then
        Me.AllowTheming = False
    End If
End Sub