Разобрался:
<Style x:Key="ListItem" TargetType="{x:Type Expander}">
<Setter Property="BorderBrush" Value="{StaticResource DefaultBorder}"/>
<Setter Property="BorderThickness" Value="0,0,0,1"/>
<Setter Property="Padding" Value="0,2"/>
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate>
<Border BorderBrush="{StaticResource DefaultBorder}" BorderThickness="0,1,0,0">
<Grid>
<Border Background="Transparent" Padding="0,10,0,10" >
<ContentControl Margin="24,0,0,0" Content="{TemplateBinding Content}"/>
</Border>
</Grid>
</Border>
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
Мне нужно было ContentControl
.:)