Thank you for writing in, please navigate to Theme Options > Typography and configure your Root Font Size properly.
This will be your base font size for various screen sizes.
For example on the small screen, you have the root font size of 14px, then the h2 are set to
Assuming that there is no other custom CSS that is conflicting this formula, so that would be (
14 * 285.7% = 39.998) 39.998px is the size of your h2 on mobile (small screen,).
Basically, that is how the Root Font Size step mode works, assuming that there is no other custom css
font-size on the way.
Hope this shed some lights,