People have deep-seated preferences regarding reading fonts, but the question of serif vs. sans-serif fonts also has an accessibility aspect. While for people with good vision, serif fonts are usually easy and a bit faster to read, sans-serif fonts tend to be harder to read for people with low vision.

https://www.section508.gov/develop/fonts-typography/

With our support for themes, it would be quite easy and unobtrusive to extend the theme switcher with a setting to switch between serif and sans-serif fonts.

To experiment with the feature I've built a prototype with a font setting added to the theme switcher:

https://cr.openjdk.org/~hannesw/font-selector/api.00/java.base/java/lang/package-summary.html