Home Header nav customization

Site URL: https://sturgeon-goby-y5ad.squarespace.com/config/

I added some css design to my home header navigation links but they dont seem to work when the css code window is closed it doesnt work when i refresh the page? any solution 
the code i implemented is as follows :

.header-nav-item:hover
  a[href="/en/our-mission"] {
      border: 3px solid #ffff;
    padding: 2px;
      border-radius: 9px;

any help please?