Hello,
You can embed a default style in TreeView's private resource dictionary, or set TreeView's ItemContainerStyle.
<TreeView>
<Treeview.resources>
<Style Targettype="{x:Type TreeViewItem}">
BasedOn="{StaticResource {x:Type TreeViewItem}}">
<Setter Property="Background" Value="Green" />
</Style>
</Treeview.resources>
<Treeviewitem header="Merry" />
<Treeviewitem header="Christmas" />
</Treeview>
<Grid.Resources>
<ResourceDictionary>
<Style x:Key="style" TargetType="{x:Type TreeViewItem}"
BasedOn="{StaticResource {x:Type TreeViewItem}}">
<Setter Property="Background" Value="Green" />
<Setter Property="ItemContainerStyle" Value="{Binding ItemContainerStyle, RelativeSource={RelativeSource AncestorType={x:Type TreeView}}}" />
</Style>
</ResourceDictionary>
</Grid.Resources>
<TreeView ItemContainerStyle="{StaticResource style}">
<TreeViewItem Header="Merry" >
<TreeViewItem Header="Christmas" />
</TreeViewItem>
</TreeView>
Regards
Joseph Leung