There’s two font sizes showing, one using the px size and the other using rem which is a relative sizing. The rem one will be the one that overrides the px because it follows right after the px. I use this tool to convert px to rem (also em) http://www.pxtoem.com but if you prefer px only, simply remove it and change the font-size for the px one.
Reminder: Whenever making modifications to core theme files, it’s recommended you use a child theme or for css only, if you have jetpack installed, it has a nice css editor that can be used so you don’t need a child theme for. But basically a child theme helps maintain any modifications a person does whenever there is an update to the main theme (parent theme) and won’t be lost on updates.