Since FormFactor doesn't have a Custom CSS area to add the CSS you need to add it another way. So Goto ...
Site Administration > Appearance > Additional HTML
You can now add the css in the box marked Head. And this is how you need to write it.
<style> .headermain button { ... ... ... } </style>
Then save these changes and go back to your site and refresh your browser using Ctrl + F5
Mary