public CornerRadius CornerRadius { get; set; }

Public Property CornerRadius As CornerRadius

<Window ...
    xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors">
    <StackPanel>
        <dxe:ToggleSwitch Margin="5" BorderThickness="2" BorderBrush="#FF757575" 
                          Background="White" CornerRadius="10"
                          ToggleSwitchHeight="20" ToggleSwitchWidth="45"
                          HorizontalAlignment="Left" ContentPlacement="Far"
                          CheckedStateContent="On" UncheckedStateContent="Off"
                          IsThreeState="True" UndefinedStateContent="Null"
                          dxe:EditorAppearanceProperties.FocusedBorderBrush="#FF6200EE"
                          dxe:EditorAppearanceProperties.CheckedStateBackground="#FF6200EE"
                          dxe:EditorAppearanceProperties.CheckedStateBorderBrush="#FF6200EE">>
            <dxe:ToggleSwitch.SwitchThumbTemplate>
                <DataTemplate>
                    <Grid>
                        <Ellipse Width="12" Height="12" Margin="5,4" Fill="#FF757575" x:Name="ellipse" />
                    </Grid>
                    <DataTemplate.Triggers>
                        <DataTrigger Binding="{Binding Path=IsChecked, RelativeSource={RelativeSource AncestorType=dxe:ToggleSwitch}}"
                                                 Value="True">
                            <Setter Property="Fill"
                                                TargetName="ellipse" Value="PaleGoldenrod" />
                        </DataTrigger>
                    </DataTemplate.Triggers>
                </DataTemplate>
            </dxe:ToggleSwitch.SwitchThumbTemplate>
        </dxe:ToggleSwitch>
    </StackPanel>
</Window>