よし、やっとやった!添付で私の解決策を見ることができ、私は結果に満足しています。誰かコメントがあれば、遠慮なくコメントを書いてください。ご助力ありがとうございます
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style TargetType="CheckBox" x:Key="CircleCheckbox">
<Setter Property="Cursor" Value="Hand"></Setter>
<Setter Property="Content" Value=""></Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CheckBox}">
<Grid>
<Ellipse x:Name="outerEllipse">
<Ellipse.Fill>
<RadialGradientBrush>
<GradientStop Offset="0" Color="Red"/>
<GradientStop Offset="0.88" Color="LightCoral"/>
<GradientStop Offset="1" Color="DarkRed"/>
</RadialGradientBrush>
</Ellipse.Fill>
</Ellipse>
<Ellipse Margin="10" x:Name="highlightCircle" >
<Ellipse.Fill >
<LinearGradientBrush >
<GradientStop Offset="0" Color="Green"/>
<GradientStop Offset="0.5" Color="LightGreen"/>
<GradientStop Offset="1" Color="DarkGreen"/>
</LinearGradientBrush>
</Ellipse.Fill>
</Ellipse>
<ContentPresenter x:Name="content" HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter TargetName="highlightCircle" Property="Fill">
<Setter.Value>
<LinearGradientBrush StartPoint="0.3,0" EndPoint="0.7,1">
<GradientStop Offset="0" Color="Green"/>
<GradientStop Offset="0.5" Color="LightGreen"/>
<GradientStop Offset="1" Color="DarkGreen"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter TargetName="outerEllipse" Property="Fill">
<Setter.Value>
<RadialGradientBrush>
<GradientStop Offset="0" Color="Green"/>
<GradientStop Offset="0.88" Color="LightGreen"/>
<GradientStop Offset="1" Color="DarkGreen"/>
</RadialGradientBrush>
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="IsChecked" Value="False">
<Setter TargetName="highlightCircle" Property="Fill">
<Setter.Value>
<LinearGradientBrush StartPoint="0.3,0" EndPoint="0.7,1">
<GradientStop Offset="0" Color="Red"/>
<GradientStop Offset="0.5" Color="LightCoral"/>
<GradientStop Offset="1" Color="DarkRed"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>