Adding to @Sheridan's answer
This isn't a 100% accurate but should get you pretty close (it's using the colors from GridView which is pretty close to Windows Explorer)

<TreeView ...>    <TreeView.Resources>        <LinearGradientBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" EndPoint="0,1" StartPoint="0,0">            <GradientStop Color="#FFD9F4FF" Offset="0"/>            <GradientStop Color="#FF9BDDFB" Offset="1"/>        </LinearGradientBrush>        <LinearGradientBrush x:Key="{x:Static SystemColors.ControlBrushKey}" EndPoint="0,1" StartPoint="0,0">            <GradientStop Color="#FFEEEDED" Offset="0"/>            <GradientStop Color="#FFDDDDDD" Offset="1"/>        </LinearGradientBrush>        <SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}" Color="Black" />        <SolidColorBrush x:Key="{x:Static SystemColors.ControlTextBrushKey}" Color="Black" />    </TreeView.Resources>    <TreeView.ItemContainerStyle>        <Style TargetType="{x:Type TreeViewItem}">            <Setter Property="BorderThickness" Value="1.5"/>            <Style.Triggers>                <Trigger Property="IsSelected" Value="True">                    <Setter Property="BorderBrush" Value="#adc6e5"/>                </Trigger>                <MultiTrigger>                    <MultiTrigger.Conditions>                        <Condition Property="IsSelected" Value="True"/>                        <Condition Property="IsSelectionActive" Value="False"/>                    </MultiTrigger.Conditions>                    <Setter Property="BorderBrush" Value="LightGray"/>                </MultiTrigger>            </Style.Triggers>            <Style.Resources>                <Style TargetType="Border">                    <Setter Property="CornerRadius" Value="2"/>                </Style>            </Style.Resources>                            </Style>    </TreeView.ItemContainerStyle></TreeView>

Add this into your TreeView.ContainerStyle to remove the default blue background.

<Style.Resources>    <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Transparent" />    <SolidColorBrush x:Key="{x:Static SystemColors.ControlBrushKey}" Color="Transparent" />    <SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}" Color="Black" />    <SolidColorBrush x:Key="{x:Static SystemColors.ControlTextBrushKey}" Color="Black" /></Style.Resources>

You can replace the Black with whatever colour you want your item text and selected item text to be.

To have a grey background when not focused, you could set up a 'non focused' Style with a grey backgorund and use EventTriggers on the TreeViewItem.GotFocus and LostFocus events to switch between the Styles.

EDIT>>>

If you want to be flash, you can use animations to change between the background colours by adding triggers to your ItemBorder Border directly in your HierarchicalDataTemplate like so:

<Border.Triggers>    <EventTrigger RoutedEvent="Border.GotFocus">        <EventTrigger.Actions>            <BeginStoryboard>                <Storyboard>                    <ColorAnimation Storyboard.TargetProperty="(Border.Background).(SolidColorBrush.Color)" To="YourColour" Duration="0:0:0.2" />                </Storyboard>            </BeginStoryboard>        </EventTrigger.Actions>    </EventTrigger>    <EventTrigger RoutedEvent="Border.LostFocus">        <EventTrigger.Actions>            <BeginStoryboard>                <Storyboard>                    <ColorAnimation Storyboard.TargetProperty="(Border.Background).(SolidColorBrush.Color)" To="LightGray" Duration="0:0:0.2" />                </Storyboard>            </BeginStoryboard>        </EventTrigger.Actions>    </EventTrigger></Border.Triggers>

Note that this will only work if the ColorAnimation has a From colour. As this code stands, the runtime will look for a SolidColorBrush set on the Border.Background property, so you must set one. You could set the ColorAnimation.From property directly instead.

Windows 10 TreeView (and ListView) Style

I was originally looking for a way to apply the Windows 10 color scheme to a TreeViewItem, including

• IsMouseOver on current item only
• Windows 10 colors which WPF already applies them to ListBox (not Windows Explorer)

If any of you are looking for exactly this, please feel free to take the code below. I used Helge Klein's solution for the IsMouseOver issue and applied the Windows 10 colors to the XAML. Therefore I propose this as an addition to the accepted answer.

Also, see below for a word on ListView and ComboBox as well.

Screenshot

App.xaml

<Style TargetType="{x:Type TreeView}">    <Style.Resources>        <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="#CBE8F6" />        <SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightBrushKey}" Color="#F6F6F6" />        <SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}" Color="Black" />        <SolidColorBrush x:Key="{x:Static SystemColors.ControlTextBrushKey}" Color="Black" />    </Style.Resources></Style><Style TargetType="{x:Type TreeViewItem}">    <Setter Property="BorderThickness" Value="1" />    <Style.Triggers>        <Trigger Property="IsSelected" Value="True">            <Setter Property="BorderBrush" Value="#26A0DA" />        </Trigger>        <MultiTrigger>            <MultiTrigger.Conditions>                <Condition Property="local:TreeViewItemHelper.IsMouseDirectlyOverItem" Value="True" />                <Condition Property="IsSelected" Value="False" />            </MultiTrigger.Conditions>            <Setter Property="Background" Value="#E5F3FB" />            <Setter Property="BorderBrush" Value="#70C0E7" />        </MultiTrigger>        <MultiTrigger>            <MultiTrigger.Conditions>                <Condition Property="IsSelected" Value="True" />                <Condition Property="IsSelectionActive" Value="False" />            </MultiTrigger.Conditions>            <Setter Property="BorderBrush" Value="#DADADA" />        </MultiTrigger>    </Style.Triggers></Style>

TreeViewItemHelper (by Helge Klein, minor changes / simplification)

public static class TreeViewItemHelper{    private static TreeViewItem CurrentItem;    private static readonly RoutedEvent UpdateOverItemEvent = EventManager.RegisterRoutedEvent("UpdateOverItem", RoutingStrategy.Bubble, typeof(RoutedEventHandler), typeof(TreeViewItemHelper));    private static readonly DependencyPropertyKey IsMouseDirectlyOverItemKey = DependencyProperty.RegisterAttachedReadOnly("IsMouseDirectlyOverItem", typeof(bool), typeof(TreeViewItemHelper), new FrameworkPropertyMetadata(null, new CoerceValueCallback(CalculateIsMouseDirectlyOverItem)));    public static readonly DependencyProperty IsMouseDirectlyOverItemProperty = IsMouseDirectlyOverItemKey.DependencyProperty;    static TreeViewItemHelper()    {        EventManager.RegisterClassHandler(typeof(TreeViewItem), UIElement.MouseEnterEvent, new MouseEventHandler(OnMouseTransition), true);        EventManager.RegisterClassHandler(typeof(TreeViewItem), UIElement.MouseLeaveEvent, new MouseEventHandler(OnMouseTransition), true);        EventManager.RegisterClassHandler(typeof(TreeViewItem), UpdateOverItemEvent, new RoutedEventHandler(OnUpdateOverItem));    }    public static bool GetIsMouseDirectlyOverItem(DependencyObject obj)    {        return (bool)obj.GetValue(IsMouseDirectlyOverItemProperty);    }    private static object CalculateIsMouseDirectlyOverItem(DependencyObject item, object value)    {        return item == CurrentItem;    }    private static void OnUpdateOverItem(object sender, RoutedEventArgs e)    {        CurrentItem = sender as TreeViewItem;        CurrentItem.InvalidateProperty(IsMouseDirectlyOverItemProperty);        e.Handled = true;    }    private static void OnMouseTransition(object sender, MouseEventArgs e)    {        lock (IsMouseDirectlyOverItemProperty)        {            if (CurrentItem != null)            {                DependencyObject oldItem = CurrentItem;                CurrentItem = null;                oldItem.InvalidateProperty(IsMouseDirectlyOverItemProperty);            }            Mouse.DirectlyOver?.RaiseEvent(new RoutedEventArgs(UpdateOverItemEvent));        }    }}

ListBox/ListView and ComboBox: In Windows 7 (and 8?), this will cause the design from TreeView to ListBox/ListView and ComboBox to differ. Therefore, if you want to apply this color scheme to these control types as well, too, use this:

<Style TargetType="{x:Type ListBoxItem}">    <Setter Property="Template">        <Setter.Value>            <ControlTemplate TargetType="{x:Type ListBoxItem}">                <Border Name="Border" BorderThickness="1" Background="Transparent">                    <ContentPresenter />                </Border>                <ControlTemplate.Triggers>                    <Trigger Property="IsMouseOver" Value="True">                        <Setter TargetName="Border" Property="Background" Value="#E5F3FB" />                        <Setter TargetName="Border" Property="BorderBrush" Value="#70C0E7" />                    </Trigger>                    <Trigger Property="IsSelected" Value="True">                        <Setter TargetName="Border" Property="Background" Value="#CBE8F6" />                        <Setter TargetName="Border" Property="BorderBrush" Value="#26A0DA" />                    </Trigger>                    <MultiTrigger>                        <MultiTrigger.Conditions>                            <Condition Property="IsSelected" Value="True" />                            <Condition Property="Selector.IsSelectionActive" Value="False" />                        </MultiTrigger.Conditions>                        <Setter TargetName="Border" Property="Background" Value="#F6F6F6" />                        <Setter TargetName="Border" Property="BorderBrush" Value="#DADADA" />                    </MultiTrigger>                </ControlTemplate.Triggers>            </ControlTemplate>        </Setter.Value>    </Setter></Style><Style TargetType="{x:Type ListViewItem}" BasedOn="{StaticResource {x:Type ListBoxItem}}" /><Style TargetType="{x:Type ComboBoxItem}">    <Setter Property="Template">        <Setter.Value>            <ControlTemplate TargetType="{x:Type ComboBoxItem}">                <Border Name="Border" BorderThickness="1" Padding="1" Background="Transparent">                    <ContentPresenter />                </Border>                <ControlTemplate.Triggers>                    <Trigger Property="IsMouseOver" Value="True">                        <Setter TargetName="Border" Property="Background" Value="#E5F3FB" />                        <Setter TargetName="Border" Property="BorderBrush" Value="#70C0E7" />                    </Trigger>                    <Trigger Property="IsSelected" Value="True">                        <Setter TargetName="Border" Property="Background" Value="#CBE8F6" />                        <Setter TargetName="Border" Property="BorderBrush" Value="#26A0DA" />                    </Trigger>                </ControlTemplate.Triggers>            </ControlTemplate>        </Setter.Value>    </Setter></Style>