[Resolved] Haven theme – How to adjust font size to a specific px value?

Hello,
I am using the Haven theme and ran into what I believe is an issue when trying to adjust font size across the whole theme.
I am using the Customize > Fonts options and it gives me the ability to change the font size when using the slider, but the numeric indicator on the right size does NOT change (always 20). So it is hard (even with the visual feedback on the page itself) to adjust font size.
Am I missing something?
I tried to use Custom Theme CSS to have fine control but it does not seem to work. Here is what I tried:

body h1 {
font-size: 24px;
}

based on: https://www.boldgrid.com/support/question/4522-how-do-i-change-all-the-default-text-sizes-eg-default-paragraph-text-is-too-big/

Any feedback would be useful.
Thanks,
Thierry