Hi, 

to change the colors of the scroll to top button kindly go to Theme Options -> Custom CSS https://prnt.sc/WWRjTlmSCjh9 and use this code: 

.scroll-top-button:before {
  background-color: #00BCD4 !important;
  color: #fff !important;
  }

for the opacity: 

.scroll-top-button.visible {
  opacity: 1 !important;
  }