lanuevacreativa

Have a look through this forum post, this topic is being discussed a lot right now!
Basically, you need to try adding !important after the font properties. Then save and refresh the page.

I see it worked here. Try checking in live mode

If you mean font size, use this code (you missing !important)
.header-menu-nav-item a { font-size: 17px !important; } if custom font, which code did you use?

The text you've posted isn't valid CSS. It won't work unless you remove the "a".
If you want to override the size of desktop nav items, you'd typically use something like this:
.header-nav-item a {   font-size: 16px; } To override mobile nav items, you can use something like this:
.header-menu-nav-item a { font-size: 6vmin; } If these don't work for you, please provide us with a working link to your website, and ideally some screenshots of what you are seeing. It's impossible for us to troubleshoot an issue on your site unless we can access the site and check the code.

remove an

Try with an !important after the 17px, inside the semicolon.
You could also try without the a. 

TBH whatever Squarespace is doing in the background has made the header text mostly impervious to CSS this evening, it's not just you.

There's a bit of a bug currently with custom CSS showing error messages when the area no errors. I think both codes look fine, and suggest ignoring the error messages for now.

I'm having the same issue.  My custom CSS header font stopped worked this morning without having any prior issues.  I tried the above suggestions but it hasn't fixed the problem unfortunately.
 
The code that I have is a very simple one:
 
.header-menu-nav-item a {
    font-size: 17px;
}
And it doesnt worked in any of my web sites

Hi,
Try the above from another poster tuanphan...the CSS works well for me