The bottom of the card that shows the validation summary is missing (overlapped by the summary visual) when the Expression_Dark theme is used.

To work this around, you can set the Margin of the DataFormValidationSummary element to 0, via an implicit style defined in the RadCardView.Resources dictionary.

<telerik:RadCardView.Resources>
    <Style TargetType="dataForm:DataFormValidationSummary">
        <Setter Property="Margin" Value="0" />
    </Style>
</telerik:RadCardView.Resources>