Applies to Expander. An expander's header is a ToggleButton, and which way it points depends on ExpandDirection — so the helper carries one header style per direction rather than one style.

Header styles

All four are Style, and all four are filled in by the Expander style — the template picks the one that matches the current direction. Replacing one only changes that direction, so an expander whose direction is bound needs the matching styles set for every value it can take.

<Expander Header="Details"
          mah:ExpanderHelper.HeaderDownStyle="{StaticResource QuietExpanderHeader}">
    <TextBlock Margin="8" Text="Expander content" />
</Expander>

Base your own on the built-in one for the direction you are replacing, or the arrow and the layout go with it:

<Style x:Key="QuietExpanderHeader"
       BasedOn="{StaticResource MahApps.Styles.ToggleButton.ExpanderHeader.Down}"
       TargetType="{x:Type ToggleButton}">
    <Setter Property="Background" Value="{DynamicResource MahApps.Brushes.Gray10}" />
    <Setter Property="Foreground" Value="{DynamicResource MahApps.Brushes.Text}" />
</Style>

Animations

The helper's own default for both is null, but the Expander style fills them in — MahApps.Storyboard.Expander.Expand and .Collapse, a quarter-second opacity fade on the content site. So an expander already animates, and setting these replaces that animation rather than adding one. Set them to {x:Null} to have the content switch over in a single frame instead.

ExpandSiteControl is read-only and exists for the template's own use; it gives the content site the storyboards are applied to.