paper-gtk-theme/Paper/gtk-3.18/widgets/_tooltips.scss

/************
 * Tooltips *
 ************/
.tooltip,
.overlay-bar {

  margin: 4px;
  border: none;
  border-radius: 3px;
  background-color: $base_color;
  color: $fg_color;

  &.background {
    background-color: $base_color;
    border: none;
    box-shadow: 0 1px 2px 0 transparentize(black, 0.7);
  }

  * { // Yeah this is ugly
    padding: 2px;
    background-color: transparent;
    color: $fg_color;
  }
  
  .csd {
    border-radius: 5px;
    box-shadow: none;
  }
}