If I understand you correct, you can do it like so:
Select any class you like, here, in my example, I selected “.full-width”, but this really doesn’t matter. After that, you see that new section opens in dropdown with title “Element ID” and below this, there is ID of the element, in your case the ID of H1.
Now click “Copy styles”.
Now find the class that you woudl like to copy styles to and select “paste styles”