Good evening Nora and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
Just to set expectations, as you might know, the scope of our support is mostly to get our wonderful customers started on the right track to customize their site and to help them in case of issues. We unfortunately do not provide complete support for customization.
With that in mind, it is usually not recommended to include the CSS directly in your functions.php file (although it is possible). If you look for the H5 tag in the code, it would look something like
<h5 style="font-size:34px;">
Let me know if that helps.
Have a great day!
Geoff B.