Change font styles for "Primary Menu' in mobile view

I’ve already replaced the words “Primary Menu’ in my Storefront child theme using this code in my functions.php file

<nav id="site-navigation" class="main-navigation" role="navigation">
		<button class="menu-toggle"><?php _e( 'New Text Here', 'storefront' ); ?></button>
		<?php wp_nav_menu( array( 'theme_location' => 'primary' ) ); ?>
	</nav><!-- #site-navigation -->

So now how do I change the font style of ‘New Text Here’? I tried using the #site-navigation selector, and the .main-navigation, too but that wasn’t the answer.

Anyone?