Changing Icon to Font Awesome Icon

Hi,

I have a sidebar of my header that pulls out from the left. I would like to use this as my cart for my shop. The icon currently shows 3 bars but I would like to change it to the FontAwesome shopping cart (fa-shopping-cart). I tried looking for the css style to change it manually but I can’t seem to find where it is. I also have a custom css plugin that overrides the theme’s css if necessary.

I would rather just try to figure out a css style that will change it so I could just add it to the custom css, rather than trying to find it in the css style.

Here is a screenshot of what I am trying to change.
https://postimg.org/image/a5lfoopf7/

Any help is greatly appreciated!