In the Crystal theme, the TitleBarBackground property is set through a style trigger to a static value. This leads to an unexpected result in the dark variation of the theme. 

As a workaround you can use the following style:

  <Style TargetType="telerik:RadRibbonView" BasedOn="{StaticResource RadRibbonViewStyle}">
            <Style.Triggers>
                <Trigger Property="IsHostedInRibbonWindow" Value="True">
                    <Setter Property="TitleBarBackground" Value="{telerik:CrystalResource ResourceKey=AlternativeBrush}"/>
                </Trigger>
            </Style.Triggers>
   </Style>