You could use:
body {
--h1-size: 1.802em;
--h2-size: 1.602em;
--h3-size: 1.424em;
--h4-size: 1.266em;
--h5-size: 1.125em;
--h6-size: 1em;
}
or the CSS here: Changing text font size doesn't change the headers font size to fix it up. Seems an oversight in the default theme, and it will be fixed in the next public release.