Zeitfresser-Wordpress-Theme/inc/blocks/font-customization/site-identity-font-size/customizer-site-identity-font-size.js

jQuery( function( $ ) {

	wp.customize('site_identity_font_size',function ( value ) {
		value.bind(function ( to ) {
		        document.body.style.setProperty('--site-identity-font-size', to+'px');
		    }
		);
	} );

} );