<StackPanel>
    <ComboBox Width="200" Background="Transparent"
    Foreground="White" BorderBrush="Transparent"
    UseLayoutRounding="True" SelectedIndex="0">
        <x:String>item 1</x:String>
        <x:String>item 2</x:String>
        <x:String>item 3</x:String>
    </ComboBox>
</StackPanel>