How to stop keyboard tab focus appearing when clicking with mouse?

I am trying to improve the accessibility of my website and want to add a visible focus when a user tabs through the site using their keyboard. I saw the following answer on the forum and added the code to my site: 

However, the keyboard focus now also shows when I click a link with the mouse too, is it possible just to have this visible focus appear when using the keyboard, and not when clicking with the mouse?