index.css

/**
 * Simple styling for an action
 *
 * Semantically speaking this class represents an action that a user
 * may take. This is commonly a button or a link.
 *
 * This class, by it self, will not offer any colors. Other classes are
 * expected to define the colors.
 *
 * An action class has four states:
 *
 *   * default
 *   * hover/focus
 *   * active
 *   * disabled
 */

.action {
  background: none;
  border: 0;
  cursor: pointer;
  display: inline-block;
  font-size: 1em;
  line-height: normal;
  margin-bottom: 0;
  padding: 4px 12px;
  text-align: center;
  text-decoration: none;
  text-transform: none;
  vertical-align: middle;
  -o-transition: background-color 50ms linear;
  -ms-transition: background-color 50ms linear;
  -moz-transition: background-color 50ms linear;
  -webkit-transition: background-color 50ms linear;
  transition: background-color 50ms linear;
}

.action.disabled,
.action[disabled] {
  cursor: default;
  opacity: 0.65;
  -ms-filter: progid:DXImageTransform.Microsoft.Alpha(Opacity=65);
  filter: alpha(opacity=65);
}