make button fixed in center bottom code example
Example: add fixed button bottom
#fixed_button{
position: fixed;
z-index: 999;
right: 0.75rem;
bottom: 0.75rem;
display: none;
width: 2.625rem;
height: 2.625rem;
border-radius: 1.875rem;
background: #4f3cda url("../images/up-arrow.png") no-repeat center 47%;
background-size: auto;
background-size: 1.125rem 1.125rem;
text-indent: -9999px;
}