When we apply backgroundcolor and Stroke from style, button stay transparent and no border are drawn.
<ContentPage.Resources>
<ResourceDictionary>
<Style x:Key="borderButton" TargetType="sb:StateButton" >
<Setter Property="Stroke" Value="{StaticResource Color2}"></Setter>
<Setter Property="StrokeThickness" Value="1"></Setter>
<Setter Property="BackgroundColor" Value="{StaticResource Color3}"></Setter>
<Setter Property="StrokeShape" Value="RoundRectangle 11,11,11,11"></Setter>
</Style>
</ResourceDictionary>
</ContentPage.Resources>
<sb:StateButton Grid.Row="1" Grid.Column="1" Style="{DynamicResource borderButton}" >
<sb:StateButton.Triggers>
<Trigger TargetType="sb:StateButton" Property="State" Value="Pressed">
<Setter Property="BackgroundColor" Value="Green" />
</Trigger>
</sb:StateButton.Triggers>
<VerticalStackLayout Padding="21,8,21,8">
<Image Source="icon_stat.png" Margin="10"></Image>
<Label VerticalOptions="Center" Text="Baisser le volume" Style="{StaticResource menuCenterLabel}"/>
</VerticalStackLayout>
</sb:StateButton>