Hello,

Please add the code to the Theme settings > Custom CSS > Global

.scrollToTop:after {
    color: red;
}
body .scrollToTop {
    -webkit-box-shadow: 0 0 5px rgba(0,0,0,1);
    box-shadow: 0 0 5px rgba(0,0,0,.17);
    background-color: rgba(255,255,255,1);

}

Change the colors as per your needs, the first code is the arrow and the second is the border.

Best Regards