Add this code to the Custom CSS field on the Settings tab of your Back to Top widget’s settings:
.global-styles,
[class*='ButtonWithStyles__StyledButton-sc'] [class^='Button__Inner-sc'] {
box-shadow: 0 2px 2px 0 rgba(0,0,0,.14), 0 1px 5px 0 rgba(0,0,0,.12), 0 3px 1px -2px rgba(0,0,0,.2) !important;
}