How to control the margins of h1-6 elements?

You do this by using the additional CSS tab in the customiser.
Use your browser inspector to identify the css context of the elements you want to style.
Documentation on css here: https://www.w3schools.com/css

Wouldn’t it be incompatible with future updates of the theme? Does that call for making a child theme or not?

If you use the customiser, then this is safe against updates, the customiser changes are saved in the database and does not affect files. However if you were to instead add to or change the css in your theme’s “style.css” or other theme file, then YES, this would require a child theme.
The golden rule is that you don’t change ANYTHING in a theme that someone else could update. This extends to the rest of WordPress actually, never change any file that someone else could update, this is why the WordPress architecture uses hook and filter functions, using these from your child theme’s “functions.php” file lets you safely work with the whole codebase.
Also it does happen that updates interact unexpectedly with your customisations, though this happens less often than one would expect.

• This reply was modified 6 years, 11 months ago by RossMitchell.

THank you