Can I template what is generated by do_action( 'generate_header' ); ?

Hi there, I hope you are doing well and staying safe!!

I am trying to add a bit of custom PHP in my theme in order to create a dark mode toggle. I can't figure out where the generate_header action is coming from and I would appreciate advice on the best practice for how to add custom PHP in that spot in my child theme. Ideally, this is where I'd like to put it: https://imgur.com/pIwckWU

Thanks very much!