Workaround: Add a border inside the Content of the AccordionItem:
<telerik:AccordionItemIsExpanded="True"HeaderText="Settings"><telerik:AccordionItem.Content><telerik:RadBorderBorderThickness="2,0,2,2"BorderColor="#CEBEE7"><VerticalStackLayoutMargin="8, 12"Spacing="8"><HorizontalStackLayoutSpacing="8"><telerik:RadCheckBox /><LabelText="Make my profile private"VerticalTextAlignment="Center" /></HorizontalStackLayout><GridColumnDefinitions="Auto, *"><telerik:RadCheckBox /><LabelGrid.Column="1"Text="Only show my posts to people who follow me"LineBreakMode="WordWrap"VerticalTextAlignment="Center"Margin="8, 0, 0, 0" /></Grid></VerticalStackLayout></telerik:RadBorder></telerik:AccordionItem.Content></telerik:AccordionItem>