/* -- START Scroll to Top icon background color-- */ 
.et_pb_scroll_top {
background-color: #696298 !important;
border: 1px solid #cdbad6 !important;
color: #cdbad6 !important;
}
/* -- START Scroll to Top icon background color-- */
Code language: CSS (css)

More options

Source: https://www.b3multimedia.ie/how-to-customize-the-back-to-top-button-in-divi/

Cirle with hover

/* Default button appearance */
.et_pb_scroll_top.et-pb-icon {
    right: 30px; /* space from right */
    bottom: 30px; /* space from bottom */
    padding: 8px; /*padding size*/
    border-radius: 30px; /* make the circle */
    background: rgb(0, 101, 253) /* button color */;	
    font-size: 32px; /* icon size */
    transition: all .1s ease-in-out; /* add transition */
}
/* Button Appearance on Mouseover */
.et_pb_scroll_top.et-pb-icon:hover {
    bottom: 32px /* space from bottom */;
    background: rgb(7, 69, 160) /*button hover color*/;	
    transition: all .1s ease-in-out; /*add transition*/
    box-shadow: 0 10px 15px #162c4e4a; /*box shadow on hover*/
}
/* Animation tweaks */
.et_pb_scroll_top.et-visible {
    -webkit-animation: fadeInBottom 1s 1 cubic-bezier(.50,0,.160,1);
    -moz-animation: fadeInBottom 2s 1 cubic-bezier(.50,0,.160,1);
    -o-animation: fadeInBottom 1s 1 cubic-bezier(.50,0,.160,1);
    animation: fadeInBottom 1s 1 cubic-bezier(.50,0,.160,1);
}
.et_pb_scroll_top.et-hidden {
    opacity: 0;
    -webkit-animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
    -moz-animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
    -o-animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
    animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
}
Code language: CSS (css)

Centered GIF Image

.et_pb_scroll_top:before {
    content: url(https://your-image-url-here.gif);/*Your image URL*/
}
.et_pb_scroll_top.et-pb-icon {
    background: none;
    right:48%;
    bottom: -7px;
}
/* Animation from Bottom */
.et_pb_scroll_top.et-visible {
    -webkit-animation: fadeInBottom 1s 1 cubic-bezier(.50,0,.160,1);
    -moz-animation: fadeInBottom 2s 1 cubic-bezier(.50,0,.160,1);
    -o-animation: fadeInBottom 1s 1 cubic-bezier(.50,0,.160,1);
    animation: fadeInBottom 1s 1 cubic-bezier(.50,0,.160,1);
}
.et_pb_scroll_top.et-hidden {
    opacity: 0;
    -webkit-animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
    -moz-animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
    -o-animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
    animation: fadeOutBottom 1s 1 cubic-bezier(.77,0,.175,1);
}
Code language: CSS (css)