Java SE (Java Platform, Standard Edition)

Hello.

I am developing an application that must be correctly displayed both in my local computer screen and in a bigger screen in production. I would like to define the font sizes across the application with CSS, in a relative way (using 'em'), supposing that the base font size is different in each screen. But I don't know how to achieve this.
When developing css for the web, I think that the 'em' is refered to the base font size established in the browser configuration. In JavaFX, how can i set this base font size? Where does JavaFX get this value from?

Thanks ind advance.

Regards.