How to easily change the font of titles and headings

In this case, I think your best bet is using the custom theme variables/properties that are built-in and ready to go. They can be found in the link and also in the app.css file in the developer tools.

The inline title picks up the --h1-font value by default, but I added that in (commented out) for if you want to change it later. Give this a try:

body {
    /* --inline-title-font: var(--h1-font); */
    --h1-font: Georgia;
    --h2-font: Georgia;
    --h3-font: Georgia;
    --h4-font: Georgia;
    --h5-font: Georgia;
    --h6-font: Georgia;
}

Have a good look over this post as well: