A custom MahApps ToggleButton style.

CustomMetroRectToggleButtonStyle.xaml

<Style x:Key="CustomMetroRectToggleButtonStyle" TargetType="{x:Type ToggleButton}">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="BorderBrush" Value="{DynamicResource GrayBrush3}" />
    <Setter Property="BorderThickness" Value="2" />
    <Setter Property="Foreground" Value="{DynamicResource BlackBrush}" />
    <Setter Property="HorizontalContentAlignment" Value="Center" />
    <Setter Property="Padding" Value="1" />
    <Setter Property="SnapsToDevicePixels" Value="True" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Grid>
                    <Border Background="{TemplateBinding Background}"
                            BorderThickness="0"
                            SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    <Border x:Name="IsCheckedBackgroundRect"
                            Background="{DynamicResource AccentColorBrush}"
                            Opacity="0"
                            BorderThickness="0"
                            SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    <Border x:Name="BorderRect"
                            Opacity="1"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    <Controls:ContentControlEx x:Name="PART_ContentPresenter"
                                                Padding="{TemplateBinding Padding}"
                                                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                                Content="{TemplateBinding Content}"
                                                ContentCharacterCasing="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(Controls:ControlsHelper.ContentCharacterCasing)}"
                                                ContentStringFormat="{TemplateBinding ContentStringFormat}"
                                                ContentTemplate="{TemplateBinding ContentTemplate}"
                                                ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"
                                                RecognizesAccessKey="True"
                                                SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="ToggleButton.IsChecked" Value="True">
                        <Trigger.EnterActions>
                            <BeginStoryboard>
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetName="IsCheckedBackgroundRect"
                                                        Storyboard.TargetProperty="Opacity"
                                                        From="0"
                                                        To="1"
                                                        Duration="0:0:0.3" />
                                </Storyboard>
                            </BeginStoryboard>
                        </Trigger.EnterActions>
                        <!--  some out fading  -->
                        <Trigger.ExitActions>
                            <BeginStoryboard>
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetName="IsCheckedBackgroundRect"
                                                        Storyboard.TargetProperty="Opacity"
                                                        To="0"
                                                        Duration="0:0:0.3" />
                                </Storyboard>
                            </BeginStoryboard>
                        </Trigger.ExitActions>
                        <Setter Property="Foreground" Value="{DynamicResource IdealForegroundColorBrush}" />
                        <Setter TargetName="BorderRect" Property="BorderBrush" Value="{DynamicResource AccentColorBrush}" />
                    </Trigger>
                    <Trigger Property="IsMouseOver" Value="False">
                        <Setter TargetName="BorderRect" Property="Opacity" Value="0.7" />
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="False">
                        <Setter Property="Foreground" Value="{DynamicResource GrayBrush7}" />
                        <Setter TargetName="BorderRect" Property="Opacity" Value="0.3" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="VerticalContentAlignment" Value="Center" />
</Style>