calibre-web/cps/static/css/popup.css

/* http://davidwalsh.name/css-tooltips */

/* base CSS element */
.popup {
  background: #eee;
  border: 1px solid #ccc;
  padding: 10px;
  border-radius: 8px;
  box-shadow: 0 2px 5px rgba(0, 0, 0, 0.2);
  position: fixed;
  max-width: 300px;
  font-size: 12px;
  display: none;
  margin-left: 2px;
  margin-top: 30px;
}

.popup.above {
  margin-top: -10px;
}

.popup.left {
  margin-left: -20px;
}

.popup.right {
  margin-left: 40px;
}

.pop_content {
  max-height: 225px;
  overflow-y: auto;
}

.pop_content > p {
  margin-top: 0;
}

/* below */
.popup::before {
  position: absolute;
  display: inline-block;
  border-bottom: 10px solid #eee;
  border-right: 10px solid transparent;
  border-left: 10px solid transparent;
  border-bottom-color: rgba(0, 0, 0, 0.2);
  left: 50%;
  top: -10px;
  margin-left: -6px;
  content: '';
}

.popup::after {
  position: absolute;
  display: inline-block;
  border-bottom: 9px solid #eee;
  border-right: 9px solid transparent;
  border-left: 9px solid transparent;
  left: 50%;
  top: -9px;
  margin-left: -5px;
  content: '';
}

/* above */
.popup.above::before {
  border-bottom: none;
  border-top: 10px solid #eee;
  border-top-color: rgba(0, 0, 0, 0.2);
  top: 100%;
}

.popup.above::after {
  border-bottom: none;
  border-top: 9px solid #eee;
  top: 100%;
}

.popup.left::before,
.popup.left::after {
  left: 20px;
}

.popup.right::before,
.popup.right::after {
  left: auto;
  right: 20px;
}

.popup.show,
.popup.on {
  display: block;
}