Override Expander Style

[Progiscad]

Hello
I would like to change the style of the expander's header. I would like to remove its rectangular appearance with a border and its CornerRadius property

Can you give me a style idea to do this?
Thank you

[timunie]

You should be able to set mah:ControlsHelper.CornerRadius (at least in upcomming v3):

MahApps.Metro/src/MahApps.Metro/Styles/Controls.Expander.xaml

Line 350 in f7ba305

CornerRadius="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(mah:ControlsHelper.CornerRadius), Converter={StaticResource CornerRadiusBindingConverter}, ConverterParameter={x:Static mahConverters:RadiusType.Bottom}}"