When the Expression_Dark theme is applied to the RadPropertyGrid, selecting an item is hard to read, as the item's foreground is white and the selection state is in a similar color.
To work this around, use the following Style:
<Style TargetType="telerik:PropertyGridField">
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Foreground" Value="Black"/>
</Trigger>
</Style.Triggers>
</Style>