Changing the color of a menu icon

Using Elgg 2.3.4 I'm trying to override the default color of an icon.  When viewing the page and checking the style in the Chrome dev console, I see the style is set to :

.fa-check:before {
  1. content: "\f00c";
}
So for now I'm just trying to update the core css file located in my /var/www/html/vendor/elgg/elgg/views/default/elements/core.css.php.  I'm adding
the value:
.fa-check:before {color:green}
 
but it's not updating.