Screen sizes and break points

Also, am I able to access the CSS of this site? If so, how do I do that?

Hi there – you can see Harmonic’s stylesheet – including media queries (mobile breakpoints) at:

https://s0.wp.com/wp-content/themes/pub/harmonic/style.css?minify=false

These forums are for folks running the WordPress software on their own hosting, not on WordPress.com. If you need further help with your theme on WordPress.com, feel free to post in the themes forum here:
https://en.forums.wordpress.com/forum/themes

Or for help with custom CSS, you can post in the CSS Customization forum:

https://en.forums.wordpress.com/forum/css-customization

Custom CSS is available with the Premium or Business plans.

Here are some tips on getting help in the WordPress.com forums: https://en.support.wordpress.com/getting-help-in-the-forums/

You can also access the live chat support option when available here:

https://wordpress.com/help/contact