paper-gtk-theme/Paper/gtk-3.0/widgets/spinner.css

/* Copyright 2016 Sam Hewitt.
*
* This file is part of the Paper GTK theme.
*
* The Paper GTK theme is free software: you can redistribute it
* and/or modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* The Paper GTK theme is distributed in the hope that it will be
* useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
* Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with the Paper GTK theme. If not, see http://www.gnu.org/licenses/.
*/

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

.menu.spinner,
.primary-toolbar .spinner {
    color: @selection;
}

/*********************
 * Spinner Animation *
 *********************/

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

.spinner {
  background-image: none;
  background: none;
  opacity: 0;
  -gtk-icon-source: -gtk-icontheme("process-working-symbolic");
}

.spinner:active {
    opacity: 1;
    animation: spin 1s linear infinite;
}
.spinner:active:insensitive {
    opacity: 0.5;
}