Font size will not change

I want to change the font-size for the widget titles and h3’s (make them smaller).

I go into the customizer, into fonts > H3 (to change the h3 font size) and fonts > Home page widget titles size (to change the widget title size). I can see the changes take place, but when I click on “save and publish”, they revert back to normal. Changes do not stick.

I am using a child theme. I’m also a newbie.

How do I get the font-size to change?