The Editor has undergone a very large visual redesign between 2018 and 2019, including using colors instead of textures to draw GUI button backgrounds. As such, both GUI.skin.button.normal.background and GUI.skin.button.active.background are identical (and null) and can't be used to implement the user's intended behavior.
There are a few solutions available as a workaround:
* Assigning any other texture to the GUIStyle.normal.background property will work as intended
* Using GUILayout.Toggle with "Button" for the style argument, which is made for this exact purpose. For example:
_pressed = GUILayout.Toggle(_pressed, _pressed ? "Change to N" : "Change to A", "Button");
* Upgrading your inspector to UI Toolkit, which has been the recommended solution for Editor content since Unity 2021