Changing project CSS

Hi guys, relatively new to Rapidweaver. I’m building a site for my photography business using the Hoa theme. I’m trying to change the navigation button appearance via css in the sidebar inspector. I’ve tried several different ways of formatting the code but can’t seem to get it right. The button is an FA button (fa fa-plus-square-o) which I’d like to change to fa fa-bars. Can anyone give me an idea as to what would be the correct code. I’ve attached a picture with the element inspector highlighting the code.

Many thanks,