Hello David
I have included disabled visual appearances for the SplitButton control in all the provided themes. This will be available in the next nightly build. If you would like to customize the disabled appearance yourself, you can create your own SplitButton style. Here is the code we use for the generic and office themes. (If you do want to modify and use this, you'll just need to include your own brush resources).
<ControlTemplate x:Key="SplitButtonToggleTemplate" TargetType="ToggleButton">
<Grid SnapsToDevicePixels="True">
<Border Background="Transparent">
<Path Fill="{TemplateBinding Foreground}" Data="M 0 0 L 8 0 L 4 4 Z" VerticalAlignment="Center" HorizontalAlignment="Center" />
</Border>
</Grid>
</ControlTemplate>
<Style x:Key="{x:Type ms:SplitButton}" TargetType="{x:Type ms:SplitButton}">
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="FocusVisualStyle" Value="{x:Null}" />
<Setter Property="Background" Value="{StaticResource ButtonBackground}" />
<Setter Property="BorderBrush" Value="{StaticResource Border}" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="Padding" Value="4" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ms:SplitButton}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="16" />
</Grid.ColumnDefinitions>
<Menu Width="10" VerticalAlignment="Bottom" HorizontalAlignment="Left">
<MenuItem IsSubmenuOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
ItemsSource="{Binding DropDownItems, RelativeSource={RelativeSource TemplatedParent}}"
Width="10" Margin="-2,0,0,-1" />
</Menu>
<Border Grid.Column="1" HorizontalAlignment="Right" Width="5" Name="Shadow" Background="{StaticResource MenuBackground}"
Visibility="Collapsed" Style="{StaticResource ShadowStyle}" />
<Border Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="2">
<Border Name="InnerBorder" BorderThickness="1" BorderBrush="{StaticResource InnerBorder}" Background="{TemplateBinding Background}" />
</Border>
<Border Name="Separator" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1,0,0,0" Grid.Column="1" Margin="0,3,0,3" />
<Border Name="PressedBorder" Margin="1,1,0,1" Opacity="0" Background="{StaticResource PressedButtonBackground}" />
<Border Name="OpenBorder" Background="{StaticResource OpenSplitButtonBackground}" Grid.ColumnSpan="2" BorderThickness="1" BorderBrush="{TemplateBinding BorderBrush}"
Visibility="{Binding IsDropDownOpen, Converter={StaticResource bvc}, RelativeSource={RelativeSource TemplatedParent}}" />
<ContentPresenter Content="{Binding Content, RelativeSource={RelativeSource TemplatedParent}}" HorizontalAlignment="Stretch"
VerticalAlignment="Stretch" Margin="{TemplateBinding Padding}" />
<ToggleButton Name="PART_ToggleButton" Grid.Column="1" Template="{StaticResource SplitButtonToggleTemplate}"
IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="InnerBorder" Property="BorderBrush" Value="{StaticResource MouseOverInnerBorder}" />
<Setter TargetName="InnerBorder" Property="Background" Value="{StaticResource MouseOverButtonBackground}" />
<Setter TargetName="Border" Property="BorderBrush" Value="{StaticResource MouseOverBorder}" />
<Setter TargetName="Separator" Property="BorderBrush" Value="{StaticResource MouseOverBorder}" />
</Trigger>
<Trigger Property="IsPressed" Value="True">
<Setter TargetName="PressedBorder" Property="Opacity" Value="1" />
<Setter TargetName="Separator" Property="Margin" Value="0" />
</Trigger>
<Trigger Property="IsDropDownOpen" Value="True">
<Setter TargetName="Shadow" Property="Visibility" Value="Visible" />
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="InnerBorder" Property="BorderBrush" Value="{StaticResource DisabledSplitButtonInnerBorder}" />
<Setter TargetName="InnerBorder" Property="Background" Value="{StaticResource DisabledSplitButtonBackground}" />
<Setter TargetName="Border" Property="BorderBrush" Value="{StaticResource DisabledSplitButtonBorder}" />
<Setter TargetName="Separator" Property="BorderBrush" Value="{StaticResource DisabledSplitButtonForeground}" />
<Setter TargetName="PART_ToggleButton" Property="Foreground" Value="{StaticResource DisabledSplitButtonForeground}" />
<Setter Property="Foreground" Value="{StaticResource DisabledSplitButtonForeground}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Jason Fauchelle