Hi Helmut,
Thank you for contacting Syncfusion support.
When using a SfSkinManager style for the window, the ItemContainerStyle will get overridden, so in order apply your custom style, you need to set the `expanderButtonStyle` to individual item instead of applying the style in ItemContainerStyle.
Also, we have modified the `expanderButtonStyle` to match up the SfSkinManager style.
Please find the modified sample below.
Please let us know if you have any concern.
Regards,
Nikhilesh