This one together with the one lynxlove mentioned could all work via css as well. For regular users just have a nice UI to change font height and font style and in the background change the CSS. And the power user does have access to all the CSS settings.
I second this - a very big advantage of ObsidianMD is the ability for users to make custom themes and CSS snippets that can also be exchanged. A custom CSS option would make it possible for CSS-proficient users to generate different styles and non-proficient users to apply styles provided by others.