customization of drop down menu

Site URL: https://garfield-law-group-demo2.squarespace.com/

PW: GONATS2020

Brand new to Squarespace.  Trying to add drop down menu (Practice-Areas) but the font size of the drop downs is too big.

Not sure how to customize the css without access to the css for the site/page.  How does one access that?

Can someone tell me the custom css to insert to reduce font size by (for example) 50% for the dropdown links?

Any help is much appreciated.

Add to Home > Design > Custom CSS

.header-nav-folder-item a {
    font-size: 14px;
}