None of them are required, call addBackToTop() without params to get nice default looks
addBackToTop({
id: 'back-to-top',
showWhenScrollTopIs: 1, // px
onClickScrollTo: 0, // px
scrollDuration: 100, // ms
innerHTML: '<strong>Go Up</strong>',
diameter: 56, // px
cornerOffset: 20, // px
backgroundColor: '#000',
textColor: '#fff',
zIndex: 1
})