Pages cut off/shortened by header

Site URL: https://www.kellysashearthing.com/

Hi all!

I'm new to Squarespace and am absolutely loving it so far.

I've set up a site for a client (see included link), and I just noticed recently that all the pages are being cut off by the header. What is the best way to approach fixing this? How do I fix it? I don't want to just add spacers; that seems like an inefficient solution. I've also read that adding too much custom code can affect SEO visibility.

Some help from more experienced folk than me would be great!

Thanks!

Add to Home > Design > Custom CSS

#page section:first-child {
    margin-top: 100px;
}