<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Grid.Resources>
    <ControlTemplate x:Key="buttonTemplate">
      <Grid>
        <Ellipse Width="100" Height="100">
          <Ellipse.Fill>
            <LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
              <GradientStop Offset="0" Color="Blue"/>
              <GradientStop Offset="1" Color="Red"/>
            </LinearGradientBrush>
          </Ellipse.Fill>
        </Ellipse>
        <Ellipse Width="80" Height="80">
          <Ellipse.Fill>
            <LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
              <GradientStop Offset="0" Color="White"/>
              <GradientStop Offset="1" Color="Transparent"/>
            </LinearGradientBrush>
          </Ellipse.Fill>
        </Ellipse>
      </Grid>
    </ControlTemplate>
  </Grid.Resources>
  <Button Template="{StaticResource buttonTemplate}">OK</Button>
</Grid>