Hello Nicklass
Here is some styling code based off our OfficeBlue theme. I wasn't sure if you mean for it to look like the SplitButton or MenuButton in the image you posted, so I went with the menu button. This will just point you in the right direction and will still need modifications to make it exactly as you want.
Search for CornerRadius and you'll find the two places where this is applied to make the main shape have rounded corners. I've commented out the separator line and related triggers. (Add these back in if you wanted it to look like a split button). The toggle button template (right at the top of the code) places the arrow on the right and gives it a slight margin. The ToggleButton within the split button is now placed across the whole split button so that the whole thing simply drops down the menu. (Change Grid.Column back to 1 on the ToggleButton if you still want the split button look).
<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="Right" Margin="0,0,4,0" />
</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" CornerRadius="4">
<Border Name="InnerBorder" BorderThickness="1" BorderBrush="{StaticResource InnerBorder}" Background="{TemplateBinding Background}" CornerRadius="3" />
</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="0" Grid.ColumnSpan="2" 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>
Let me know if you have questions about modifying and using this code.
-Jason Fauchelle
