Font size of Primary Navigation Menu

Hello, I have been searching for ways to increase the font size of the primary navigation menu label on my site, using Sinatra Theme. I did not see that the theme was no longer being maintained before building my site.

The font size appearance designation would apply to both the Menu Items as well as the Mobile Menu Button Label when it is just a hamburger in mobile view. I was able to get the size to increase once by adding html (<font size=36> and <h1>) to the menu label field in the customizer, but it reverted and no longer shows increased font size after closing out the Customizer. I have searched for custom CSS to add but what I have found and tried for this also does work for me. The only workaround I have found is to increase the Base Typography under General Settings for the whole site to the maximum font size and then going through every other part of the site and reducing the text size. If I am missing something, please help! This seems like a basic design need for a theme.

The page I need help with: [log in to see the link]