gnome-shell/data/theme/gnome-shell-sass/widgets/_corner-ripple.scss

/* Activities Ripple */

$ripple_size: 50px;

.ripple-box {
  background-color: lighten(transparentize($selected_bg_color, 0.7), 40%);
  box-shadow: 0 0 2px 2px lighten($selected_bg_color, 20%);
  // plus + 2px for the border (box-shadow)
  width: $ripple_size + 2px;
  height: $ripple_size + 2px;
  border-radius: 0 0 $ripple_size + 2px 0; // radius equals the size of the box to give us the curve

  // just a simple change to the border radius position
  &:rtl { border-radius: 0 0 0 $ripple_size + 2px; }
}

// Pointer location
.ripple-pointer-location {
  width: $ripple_size;
  height: $ripple_size;
  border-radius: $ripple_size * 0.5; // radius equals the size of the box to give us the curve
  background-color: lighten(transparentize($selected_bg_color, 0.7), 30%);
  box-shadow: 0 0 2px 2px lighten($selected_bg_color, 20%);
}