Use safer `add_theme_constant_override()` in MarginContainer code sample
Use safer `add_theme_constant_override()` in MarginContainer code sample

Control has magic setters to set custom theme items, but using the
dedicated Control methods is less prone to typos so it should be favored.

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot