button background when get foucus
<Button Content="Click Me" Background="Blue">
<Button.Template>
<ControlTemplate TargetType="{x:Type Button}">
<Border Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
<ContentPresenter Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}" Margin="{TemplateBinding Padding}" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Button.IsFocused" Value="True">
<Setter TargetName="Border" Property="Background" Value="White" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Button.Template>
</Button>