WPF ControlTemplate: How to provide a default value for TemplateBinding?

You can just define setters on your style for the two properties in question.

For example, some general definitions:

<LinearGradientBrush x:Key="ButtonNormalBackground" EndPoint="0,1" StartPoint="0,0">
    <GradientStop Color="#F3F3F3" Offset="0"/>
    <GradientStop Color="#EBEBEB" Offset="0.5"/>
    <GradientStop Color="#DDDDDD" Offset="0.5"/>
    <GradientStop Color="#CDCDCD" Offset="1"/>
</LinearGradientBrush>

<SolidColorBrush x:Key="ButtonNormalBorder" Color="#FF707070"/>

Then, in your style definition:

<Setter Property="Background" Value="{StaticResource ButtonNormalBackground}" />
<Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}" />