7.1 How do you add a horizontal line separating the Header and first section?

Site URL: http://www.oliomakes.com

Hey! 

I'm wanting to add in a horizontal line directly under the header to separate it from the first section on each page. Anybody got any ideas?

If so, can I trouble you for a follow-up question on how to have that line on only certain pages and not all of them? 

I've searched online, but only found an old code for Montauk that didn't work. I'm new to adding custom CSS so speak plainly 😁  

Thanks!