You may format the default Expander to give it's header a different view (font style, name, weight etc) and then style its contents differently to give the header a different look than the contents.
The following XAML code styles Expander header with font size 20 and foreground color green while the contents are black with font size 14.
<Window x:Class="ExpanderControlSample.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300">
<Grid>
<Expander Name="ExpanderControl" Background="LavenderBlush"
HorizontalAlignment="Left" Header="Click to Expand"
ExpandDirection="Down" IsExpanded="False" Width="200"
FontSize="20" FontWeight="Bold" Foreground="Green">
<TextBlock TextWrapping="Wrap" FontSize="14" FontWeight="Light" Foreground="Black">
This is an Expander control.
Within this control, all contents will be wrapped.
At run-time, you may expand or
collapse this control. Type more text here to be typed.
Jump around and hype.
</TextBlock>
</Expander>
</Grid>
</Window>