Need of CSS help

Hello I love this plugin!

I need some css help, I need a fixed button so it scrolls with the site

I have the topbar “on” but only want a button to show not the enire topbar or the the space the topbar adds to my site

You can see it here
https://greaterthanthesum.com/

I have tryed but not getting the proper css

Thank you

https://www.ads-software.com/plugins/shiftnav-responsive-mobile-menu/