Bind the Fill with Background
property of the Button
,
<Style x:Key="ButtonStyle_base" TargetType="{x:Type Button}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Grid Name="grid" Margin="0,0,0,0">
<Rectangle Name="rectangle" HorizontalAlignment="Stretch" Fill="{TemplateBinding Background}" VerticalAlignment="Stretch" >
<Rectangle.Effect>
<DropShadowEffect BlurRadius="3" Opacity="0.4" ShadowDepth="6"/>
</Rectangle.Effect>
</Rectangle>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsFocused" Value="True" />
<Trigger Property="IsDefaulted" Value="True" />
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Set the Background from your derived Style,
<Style x:Key="ButtonStyle" BasedOn="{StaticResource ButtonStyle_base}">
<Setter Property="Background" >
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1" >
<GradientStop Color="#FFFFFFFF" Offset="0.5" />
<GradientStop Color="#CCFFFFDD" Offset="1" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Style>