Two implicit styles, one for the tree and one for its nodes, applied by Styles/Controls.xaml.

<TreeView>
    <TreeViewItem Header="Documents" IsExpanded="True">
        <TreeViewItem Header="Invoices" IsSelected="True" />
        <TreeViewItem Header="Contracts" />
    </TreeViewItem>
    <TreeViewItem Header="Pictures">
        <TreeViewItem Header="Holiday" />
    </TreeViewItem>
    <TreeViewItem Header="Music" />
</TreeView>

The node states — the two selection colours, hover, disabled — are the same ItemHelper brushes with the same defaults as a ListBox, so that page covers them. What follows is what a tree adds.

The selection spans the row

Look at the figure: the highlight behind Invoices runs the full width of the tree, indent and all, not just behind the text. The node template wraps the whole row in the border that carries the background, which is what most file managers do and what makes a deep tree readable.

It also means HorizontalContentAlignment on the tree reaches the nodes — the style binds it from the ancestor ItemsControl, defaulting to Stretch.

No border

As with ListBox, BorderThickness is 0 — the same <!-- default to 0 --> in the source — while BorderBrush is MahApps.Brushes.ThemeForeground. Switching the frame on without also setting the brush gives a black box:

<TreeView BorderThickness="1"
          BorderBrush="{DynamicResource MahApps.Brushes.Control.Border}"
          mah:ControlsHelper.CornerRadius="4" />

A rounded corner is bitten out at the top for the same reason it is on a ListBox — the template root is a plain Border and the opaque item background covers the arc. That page has the two workarounds.

Indentation

Nodes are indented by a TreeViewMarginConverter with Length="12", which walks up the parent chain and multiplies. Twelve units per level is baked into the converter instance in the dictionary, so changing it means replacing the node template — there is no property for it.

The expander button

The little triangle is a ToggleButton bound two-way to IsExpanded, and the style it uses comes from an attached property, so it can be replaced without touching the node template:

<TreeView.ItemContainerStyle>
    <Style BasedOn="{StaticResource MahApps.Styles.TreeViewItem}" TargetType="{x:Type TreeViewItem}">
        <Setter Property="mah:TreeViewItemHelper.ToggleButtonStyle" Value="{StaticResource MyExpander}" />
    </Style>
</TreeView.ItemContainerStyle>

The default is MahApps.Styles.ToggleButton.TreeViewItem.ExpandCollapse, a filled triangle from the TreeArrow geometry that rotates when the node opens. The right-hand figure replaces it with plus and minus glyphs from the symbol font:

<Style x:Key="MyExpander" TargetType="{x:Type ToggleButton}">
    <Setter Property="Focusable" Value="False" />
    <Setter Property="Width" Value="20" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Border Background="Transparent">
                    <TextBlock x:Name="Glyph"
                               HorizontalAlignment="Center"
                               VerticalAlignment="Center"
                               FontFamily="{DynamicResource MahApps.Fonts.Family.SymbolTheme}"
                               FontSize="12"
                               Foreground="{DynamicResource MahApps.Brushes.Accent}"
                               Text="&#xE710;" />
                </Border>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsChecked" Value="True">
                        <Setter TargetName="Glyph" Property="Text" Value="&#xE738;" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Background="Transparent" on the border matters — without it only the glyph itself answers clicks, and hitting a 12-unit triangle is fiddly. See TreeViewItemHelper.

A node with no children hides the button rather than drawing a disabled one: the template has a HasItems="False" trigger for that, which is why Music in the figure has no triangle.

Virtualisation

A tree is the control where this matters most, because every expanded branch adds nodes:

<TreeView Style="{StaticResource MahApps.Styles.TreeView.Virtualized}" ItemsSource="{Binding Roots}" />

The variant sets VirtualizingStackPanel.IsVirtualizing and VirtualizationMode="Recycling". The base style watches the first of those and swaps its ItemsPanel for a VirtualizingStackPanel and CanContentScroll to True when it sees it, so setting the property by hand works too.

Note that the node style carries the same trigger for its own child panel — a tree has to virtualise at every level, not just the root.

Related

ListBox shares the item brushes, and ItemHelper documents all of them.