Custom CCS working in editor but doesn't appear on the live site

Site URL: https://weselect.com/

So I've been trying to add custom css to our Secondary Navigation to make one of the two buttons outlined. This works fine in the editor but once I load the live site the CSS is not applied. Is there something I'm doing wrong?

This is my Custom CSS:

#yui_3_17_2_1_1580978748119_1195 > div > a:nth-child(1)  {
    color: #213651!important;
    background-color: #ffffff!important;
    border: 1px solid!important;
    border-color: #213651!important;
  }

It turns out like this:

YUI IDs like the one in your screenshot are dynamically generated on page load. You shouldn't use these in your code because they will change next time the page is refreshed or edited. I published a guide that explains this here: In Squarespace, what are the differences between #block-yui and #yui selectors?