The problem is probably in the way your fonts are being enqueued in the functions.php file of your theme. Look for instances of wp_register_style and see if you find the google fonts api there with http. It’s possible that your theme has its customizer settings in a different php file, so you should probably also check for a file called customizer.php or theme-mods.php or so.
Beware that if you change theme files and the theme is updated, you will loose your modifications. The royal way would be to create a child theme.