The global styles editor can be used to modify the CSS of blocks, but not some HTML elements.

e.g. In the Twenty Twenty-Two theme I want to change the H1 font to Arial.
Can't do it in the WP5.9 global style editor.

I've got 4 options:

Use customize.php and "!important"
Use a style.css file in a child theme css file and "!important"
Use a custom css plugin and "!important"
Use a theme.json file in a child theme

Options 1-3 have to use "!important" because the Twenty Twenty-Two global styles are written at the bottom of the page, and these options write css into the section.

If option 3 allows the custom css to be written just before the tag, then "!important" is not needed.