Hmm, I'm not sure if I understand your question.
The appearance change of buttons is usually set up in the Nextion Editor and does not require any action of the MCU.
But the logic for the MCU to respond to the press/release (push/pop in ITEAD jargon) events is already part of the original ITEAD lib which I just ported and extended (or am in the middle of).
How to do that can be seen in the samples provided with the lib.
But if you want to discuss some more about the Nextion lib, I could split this thread and create a new topic for this.