I have a segmented MenuTool and below is current control template:

<ControlTemplate x:Key="MenuButtonAreaSegmented" TargetType="{x:Type igRibbon:MenuButtonArea}">

<Grid x:Name="root">
<Grid.RowDefinitions>
<RowDefinition x:Name="row0" Height="*" />
<RowDefinition x:Name="row1" Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>

<Image x:Name="imgBackground"
Grid.Column="0"
Grid.ColumnSpan="2"
Grid.Row="0"
Grid.RowSpan="3"
Source="/WpfApplication2;component/Resources/default.png"
SnapsToDevicePixels="True"
MaxHeight="26"
Height="26"
Width="29"
Stretch="UniformToFill"
Visibility="Collapsed" />

<ToggleButton x:Name="PART_SegmentedButton"
Margin="2"
Command="{x:Static igRibbon:MenuTool.SegmentedButtonCommand}"
Opacity="1"
Style="{StaticResource PlainContentControlStyle}">
<ToggleButton.Content>
<!--<igRibbon:RibbonButtonChrome x:Name="buttonChrome"
ClipToBounds="True"
IsPressed="{Binding Path=IsPressed,
ElementName=PART_SegmentedButton}"
IsSegmentedButton="True"
SnapsToDevicePixels="True">-->
<Grid x:Name="btnChromeChild"
Margin="1"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>

<ColumnDefinition x:Name="col0" Width="Auto" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>

<!-- Background="{StaticResource BaseSelectedBrush}" -->

<igWindows:SynchronizedSizeDecorator x:Name="syncImage"
SynchronizeHeight="False"
SynchronizeWidth="False"
Target="{Binding ElementName=syncText}">
<igWindows:AutoDisabledImage x:Name="image"
Width="16"
Height="16"
Margin="2"
igRibbon:XamRibbon.KeyTipPlacementType="SmallImage"
igWindows:Utilities.SnapElementToDevicePixels="True"
Source="{Binding Path=MenuTool.ImageResolved,
RelativeSource={x:Static RelativeSource.TemplatedParent}}"
Visibility="{Binding Path=MenuTool.HasImage,
RelativeSource={x:Static RelativeSource.TemplatedParent},
Converter={StaticResource BoolToVisConverter}}" />
<!--<igWindows:AutoDisabledImage x:Name="image1"
Width="16"
Height="16"
Margin="2"
igRibbon:XamRibbon.KeyTipPlacementType="SmallImage"
igWindows:Utilities.SnapElementToDevicePixels="True"
Source="{Binding RelativeSource={RelativeSource TemplatedParent},
Path=MenuTool.(behaviors:HoverSmallLargeImage.LargeHoverImage)}"
Visibility="Collapsed" />-->
</igWindows:SynchronizedSizeDecorator>
<ContentPresenter x:Name="textSmall"
Grid.Column="1"
Margin="0,0,0,0"
VerticalAlignment="Center"
igRibbon:RibbonGroupPanel.SizingModeVersion="{TemplateBinding igRibbon:RibbonGroupPanel.SizingModeVersion}"
Content="{Binding Path=MenuTool.Caption,
RelativeSource={x:Static RelativeSource.TemplatedParent},
Converter={StaticResource stringWithoutMnemonicsConverter}}"
SnapsToDevicePixels="True"
TextBlock.TextAlignment="Left" />
</Grid>
<!--</igRibbon:RibbonButtonChrome>-->
</ToggleButton.Content>
</ToggleButton>

<igRibbon:DropDownToggle x:Name="dropDownBtn"
Margin="-2,0,0,0"
Grid.Column="1"
IsDroppedDown="{Binding Path=MenuTool.IsOpen,
RelativeSource={x:Static RelativeSource.TemplatedParent}}"
Style="{StaticResource PlainContentControlStyle}">
<igRibbon:DropDownToggle.Content>
<igRibbon:RibbonButtonChrome x:Name="dropdownChrome"
ClipToBounds="True"
IsChecked="{Binding Path=IsDroppedDown,
ElementName=dropDownBtn}"
IsPressed="{Binding Path=IsPressed,
ElementName=dropDownBtn}"
Panel.ZIndex="1"
SnapsToDevicePixels="True">
<Border Name="border" BorderThickness="0" CornerRadius="3" BorderBrush="Transparent">
<Grid x:Name="dropdownPanel" HorizontalAlignment="Center">
<Grid.ColumnDefinitions>

<ColumnDefinition x:Name="dropDownCol" />
</Grid.ColumnDefinitions>
<Control x:Name="dropdownArrow"
Margin="1,0,0,0"
igRibbon:XamRibbon.KeyTipPlacementType="DropDownButton"
Style="{DynamicResource {x:Static igRibbon:MenuTool.MenuToolDropDownArrowStyleKey}}" />
<igWindows:SynchronizedSizeDecorator x:Name="syncText"
SynchronizeHeight="False"
SynchronizeWidth="False"
Target="{Binding ElementName=syncImage}">
<igRibbon:LargeToolCaptionPresenter x:Name="textLarge"
igRibbon:RibbonGroupPanel.SizingModeVersion="{TemplateBinding igRibbon:RibbonGroupPanel.SizingModeVersion}"
Text="{Binding Path=MenuTool.Caption,
RelativeSource={x:Static RelativeSource.TemplatedParent},
Converter={StaticResource stringWithoutMnemonicsConverter}}"
Visibility="Collapsed">
<igRibbon:LargeToolCaptionPresenter.Glyph>
<Control Margin="0,4,0,0" Style="{DynamicResource {x:Static igRibbon:MenuTool.MenuToolDropDownArrowStyleKey}}" />
</igRibbon:LargeToolCaptionPresenter.Glyph>
</igRibbon:LargeToolCaptionPresenter>
</igWindows:SynchronizedSizeDecorator>
</Grid>
</Border>
</igRibbon:RibbonButtonChrome>
</igRibbon:DropDownToggle.Content>
</igRibbon:DropDownToggle>
<Border x:Name="ToggleBG"
Grid.Row="1"
Grid.ColumnSpan="2"
HorizontalAlignment="Stretch"
Opacity="0" />

</Grid>
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding ElementName=dropDownBtn, Path=IsDroppedDown}" Value="true">
<!--<Setter TargetName="root" Property="Background" Value="#FF2DA5DA" />-->
<Setter Property="Foreground" Value="{DynamicResource {x:Static igRibbon:RibbonBrushKeys.TextHottrackFillKey}}" />
<!--<Setter TargetName="image1" Property="Visibility" Value="Visible" />
<Setter TargetName="image" Property="Visibility" Value="Collapsed" />-->
</DataTrigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Foreground" Value="{DynamicResource {x:Static igRibbon:RibbonBrushKeys.TextHottrackFillKey}}" />
<!--<Setter TargetName="image1" Property="Visibility" Value="Visible" />-->
<!--<Setter TargetName="image" Property="Visibility" Value="Collapsed" />-->
<Setter TargetName="imgBackground" Property="Visibility" Value="Visible" />
<Setter TargetName="imgBackground" Property="Source" Value="/WpfApplication2;component/Resources/hover_seg.png" />
<!--<Setter TargetName="root" Property="Background" Value="#FF2DA5DA" />
<Setter TargetName="ToggleBG" Property="BorderBrush" Value="White" />
<Setter TargetName="ToggleBG" Property="BorderThickness" Value="0,0,0,1" />-->
<Setter TargetName="border" Property="Background" Value="Transparent"/>
<Setter TargetName="border" Property="BorderBrush" Value="Yellow"/>
</Trigger>

<DataTrigger Binding="{Binding Path=IsPressed, ElementName=PART_SegmentedButton}" Value="true">
<Setter TargetName="imgBackground" Property="Visibility" Value="Visible" />
<Setter TargetName="imgBackground" Property="Source" Value="/WpfApplication2;component/Resources/pressed.png" />
</DataTrigger>

<Trigger SourceName="PART_SegmentedButton" Property="IsEnabled" Value="False">
<Setter TargetName="textSmall" Property="TextElement.Foreground" Value="{DynamicResource {x:Static igRibbon:RibbonBrushKeys.ToolDisabledForegroundTextFillKey}}" />
</Trigger>

<Trigger SourceName="dropDownBtn" Property="IsEnabled" Value="False">
<Setter TargetName="textLarge" Property="TextElement.Foreground" Value="{DynamicResource {x:Static igRibbon:RibbonBrushKeys.ToolDisabledForegroundTextFillKey}}" />
</Trigger>

<!--<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=IsCheckable}" Value="true" />
<Condition Binding="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=MenuTool.IsChecked}" Value="true" />
</MultiDataTrigger.Conditions>
<Setter TargetName="image1" Property="Visibility" Value="Visible" />
</MultiDataTrigger>-->
<Trigger Property="IsCheckable" Value="True">
<Setter TargetName="PART_SegmentedButton" Property="IsChecked" Value="{Binding Path=MenuTool.IsChecked, RelativeSource={x:Static RelativeSource.TemplatedParent}, Mode=OneWay}" />
<Setter TargetName="ToggleBG" Property="Opacity" Value="1" />
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="SizingMode" Value="ImageOnly" />
<Condition Property="HasImage" Value="True" />
</MultiTrigger.Conditions>
<Setter TargetName="textSmall" Property="Visibility" Value="Collapsed" />
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="SizingMode" Value="ImageAndTextLarge" />
<Condition Property="Location" Value="Ribbon" />
</MultiTrigger.Conditions>
<Setter TargetName="dropdownArrow" Property="Visibility" Value="Collapsed" />
<Setter TargetName="textSmall" Property="Visibility" Value="Collapsed" />
<Setter TargetName="textLarge" Property="Visibility" Value="Visible" />
<!--<Setter TargetName="image1" Property="Width" Value="32" />
<Setter TargetName="image1" Property="Margin" Value="0,0,0,2" />
<Setter TargetName="image1" Property="Height" Value="32" />
<Setter TargetName="image1" Property="HorizontalAlignment" Value="Center" />-->

<Setter TargetName="image" Property="Width" Value="32" />
<Setter TargetName="image" Property="Height" Value="32" />
<Setter TargetName="image" Property="Margin" Value="0,0,0,2" />
<Setter TargetName="image" Property="HorizontalAlignment" Value="Center" />
<!--<Setter TargetName="syncImage" Property="SynchronizeWidth" Value="True" />-->
<Setter TargetName="syncText" Property="SynchronizeWidth" Value="True" />
<Setter TargetName="dropDownCol" Property="Width" Value="Auto" />

<Setter TargetName="dropDownBtn" Property="Grid.Column" Value="0" />
<Setter TargetName="dropDownBtn" Property="Grid.Row" Value="2" />
<Setter TargetName="PART_SegmentedButton" Property="Grid.RowSpan" Value="2" />
<Setter TargetName="row0" Property="Height" Value="Auto" />
<Setter TargetName="row1" Property="Height" Value="*" />
<Setter TargetName="btnChromeChild" Property="Margin" Value="1,1,1,0" />
</MultiTrigger>
<DataTrigger Binding="{Binding Path=MenuTool.(igRibbon:XamRibbon.HideAccessKey), RelativeSource={x:Static RelativeSource.TemplatedParent}}" Value="False">
<Setter TargetName="textSmall" Property="RecognizesAccessKey" Value="True" />
<Setter TargetName="textSmall" Property="Content" Value="{Binding Path=(igRibbon:ButtonTool.Caption), RelativeSource={x:Static RelativeSource.TemplatedParent}}" />
</DataTrigger>

<!--<Trigger Property="Location" by Tommy
Value="QuickAccessToolbar">
-->
<!--<Setter Property="Margin" Value="0,-2,0,0"/>-->
<!--
<Setter Property="Visibility"
Value="Visible"
TargetName="image1" />
<Setter Property="Visibility"
Value="Collapsed"
TargetName="image" />
-->
<!--<Setter Property="Padding" TargetName="buttonChrome" Value="0,2,0,0"/>-->
<!--<Setter Property="Padding" TargetName="dropdownChrome" Value="0,2,0,0"/>-->
<!--
<Setter Property="Foreground"
Value="{DynamicResource {x:Static igRibbon:RibbonBrushKeys.ToolInQATForegroundTextFillKey}}" />
</Trigger>-->

</ControlTemplate.Triggers>
</ControlTemplate>

when i mouse over the dropdown toggle, there always a border which i cannot figure out where it comes from (my guess is RibbonButtonChrome: dropdownChrome)

I cannot make any changes for that border (borderbrush, thickness or corners). Any suggestions on it plz?