Use Margin or Padding, applied to the scope within the container:
<StackPanel>
<StackPanel.Resources>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Margin" Value="0,10,0,0"/>
</Style>
</StackPanel.Resources>
<TextBox Text="Apple"/>
<TextBox Text="Banana"/>
<TextBox Text="Cherry"/>
</StackPanel>
EDIT: In case you would want to re-use the margin between two containers, you can convert the margin value to a resource in an outer scope, f.e.
<Window.Resources>
<Thickness x:Key="tbMargin">0,10,0,0</Thickness>
</Window.Resources>
and then refer to this value in the inner scope
<StackPanel.Resources>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Margin" Value="{StaticResource tbMargin}"/>
</Style>
</StackPanel.Resources>
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