Are there CSS classes for GP’s heading styles? Say I want to a h2 to look like h3 because it has better readability but want the tag to remain h2 for accessibility. I know there are a few utility classes in GP already such as screen-reader-text and smooth-scroll. Is there something that would allow me to do <h2 class="h3">Hello world!</h2>, without having to duplicate my h3 customizer settings in custom CSS.
No, there are no utility classes ( its something we may revisit ).
What you can do in the Customizer > Typography is choose Custom in the Target Element List. This provides a field to add your CSS Selector eg. .h3 or it accepts multiples so you an do h3, .h3 to style both together.