Customizing CSS
If you are using a child theme and have defined :root vars in styles.css, you may find that the CSS vars defined there don't always propagate into Divi's CSS, so the best place for global CSS in Divi is through
Dashboard → Divi → Theme Options | General | Custom CSS.More specific CSS can be applied to specific modules through the builder.
Header and Body Text Sizing
Variables
The base font size is set to 16px, which is the default font size on cell phones, tablets, and most WordPress and other content management systems. Regular body text is written by default at the base font size.
Headers begin with the smallest (H6) which is also at the --base_font_size, and increase by a factor of --header_step_size with each step from H5 to H1 (the largest).