Default font size

Use child theme.

https://github.com/elementor

@bawean
Tutorial
Just look at the style file of the parent theme and repeat the desired properties in the child theme.
Or you can use the code to insert the child theme functions into the file.

function add_google_fonts() {
    if ( !is_admin() ) {
        wp_register_style('google-open-sans', 'https://fonts.googleapis.com/css?family=Open+Sans:400,700&subset=cyrillic', array(), null, 'all');
        wp_register_style('google-roboto', 'https://fonts.googleapis.com/css?family=Roboto:400,700&subset=cyrillic', array(), null, 'all');
        wp_enqueue_style('google-open-sans');
        wp_enqueue_style('google-roboto');
    }
}
add_action('wp_enqueue_scripts', 'add_google_fonts');