正如emocht提到的,我忘记在ControlTemplate中指定TargetType:
<ControlTemplate TargetType="Button">
<Border Background="#F6F9FC" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="4"
CornerRadius="20" x:Name="Border" Padding="22 0">
<ContentPresenter Content="{TemplateBinding Content}" />
</Border>
</ControlTemplate>