Update file index.scss

docs/.vuepress/styles/index.scss

@@ -29,7 +29,7 @@ h1, h2, h3, h4, h5, h6, p, a, span, strong {
     border: 0; /* Optional: removes the iframe border */
   }
   
-.vp-back-to-top-button {
+.back-to-top-icon {
   background-image: url('https://static.igem.wiki/teams/5306/natronaut-logo-no-text.png') !important;
   background-size: contain;
   background-repeat: no-repeat;