Can’t change h1 text size for #page-sub-header in child theme

I am having trouble figuring out how to change styles for the WP Bootstrap Starter theme. I’ve created a functions file with the following:

<?php
function my_theme_enqueue_styles() {

    $parent_style = 'parent-style'; // This is 'twentyfifteen-style' for the Twenty Fifteen theme.

    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style ),
        wp_get_theme()->get('Version')
    );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );
?>

I can change some parts of the theme css like blog post title font size. But I cannot change the sub header font size where it says “WordPress + Bootstrap”. I’ve tried:

#page-sub-header {
    font-size: 1rem;
}

That should have done it, but it doesn’t do anything. Driving me crazy.

Anyone have any ideas?