In your theme you can just go to the footer HTML and throw in something like this:
<style>
/*Floating Back-To-Top Button*/
#myBtn {
position: fixed;
bottom: 10px;
float: right;
right: 5%;
left: auto;
width: auto;
font-size: 12px;
border-color: rgba(85, 85, 85, 0.2);
background-color: rgb(100,100,100);
padding: 10px 15px;
border-radius: 4px;
/*On Hover Color Change*/
#myBtn:hover {
background-color: #7dbbf1;
}
</style>
<button id="myBtn"><a href="#top" style="color: white"><i class="fa-solid fa-circle-chevron-up"></i> Back to Top</a></button>