paper-gtk-theme/Paper/gtk-3.20/widgets/_spinner.scss

/***********
 * Spinner *
 ***********/

menu spinner {
    color: $selected_bg_color;
}

/*********************
 * Spinner Animation *
 *********************/
@keyframes spin {
  to { -gtk-icon-transform: rotate(1turn); }
}

.spinner,
spinner {
  background-image: none;
  background: none;
  opacity: 0; // non spinning spinner makes no sense
  -gtk-icon-source: -gtk-icontheme('process-working-symbolic');

  &:active,
  &:checked {
    opacity: 1;
    animation: spin 1s linear infinite;

    &:disabled { opacity: 0.5; }
  }
}