The template is in the toolkit here.
You might be able to get what you want by changing these button styles to add the Margin
:
<!-- DecrementButtonStyle -->
<Style
x:Key="DecrementButtonStyle"
BasedOn="{StaticResource NumericUpDownButtonStyle}"
TargetType="RepeatButton">
<Setter
Property="Content"
Value="➖" />
<Setter
Property="Margin"
Value="5" />
</Style>
<!-- IncrementButtonStyle -->
<Style
x:Key="IncrementButtonStyle"
BasedOn="{StaticResource NumericUpDownButtonStyle}"
TargetType="RepeatButton">
<Setter
Property="Content"
Value="➕" />
<Setter
Property="Margin"
Value="5" />
</Style>