Have you tried using this method –https://codex.www.ads-software.com/Creating_a_Favicon? I am using the Brunelleschi theme, too, and did this yesterday (after much trial and error) and it worked. Make sure the image you are using is actually named favicon.ico. (I had a blond moment and tried using an image named favicon2.ico the first time around.)
I play it safe when it comes to editing anything (because quite frankly I am pretty ditzy) and always copy and paste the original in a notepad (plain text) file in case what I am doing screws it up (which happens embarrassingly often). When I messed up, I just copy and paste the original right back in.
Hope this helps.
