None of them are required, call addBackToTop() without params to get nice default looks
addBackToTop({
backgroundColor: '#000',
cornerOffset: 20, // px
diameter: 56, // px
ease: inOutSine, // any one from https://www.npmjs.com/package/ease-component will do
id: 'back-to-top',
innerHTML: '<svg viewBox="0 0 24 24"><path d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path></svg>',
onClickScrollTo: 0, // px
scrollContainer: document.body, // or a DOM element, e.g., document.getElementById('content')
scrollDuration: 100, // ms
showWhenScrollTopIs: 1, // px
size: diameter, // alias for diameter
textColor: '#fff',
zIndex: 1
})