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

/**********
 * Switch *
 **********/
GtkSwitch {
  -GtkSwitch-slider-width: 20px;
  -GtkSwitch-slider-height: 20px;

  font: 1;
  font-weight: bold;
  outline-offset: -4px;
  transition: all 200ms ease-in;

  border: none;
  border-radius: 16px;
  color: transparent;

  background-color: transparentize(black,0.9);
  box-shadow: if($variant == 'light', (inset 0 0 0 1px transparentize(black,0.9), 0px 1px $bottom_highlight), (inset 0 0 0 1px transparentize(black,0.75), 0px 1px $bottom_highlight));

  &:insensitive {
    background-color: $insensitive_base_color;
    box-shadow: if($variant == 'light', inset 0 0 0 1px transparentize(black, 0.9), 0 0 0 1px transparentize(black, 0.8));
  }

  &:active,
  &:checked {
    background-color: $selected_bg_color;
    box-shadow: if($variant == 'light', inset 0 0 0 1px $selected_borders_color, 0 0 0 1px transparentize(black, 0.8));

    &:backdrop,
    &:insensitive {
      background-color: $backdrop_selected_bg_color;
    }
  }

  // Handle
  .slider {
    border:none;
    padding:2px;
    outline-radius: 12px;
    border-radius: 12px;
    transition: $button_transition;
    background-color: $base_color;

    box-shadow: inset 0 1px 0 0 transparentize(white,0.9),
                0 2px 2px 0 $shadow_color,
                0 1px 2px 0 $shadow_color;

    &:active {
      background-color: white;
    }

    &:backdrop {
      background-color: $backdrop_base_color;
      box-shadow: 0 1px 2px 0 transparentize(black, 0.9);
    }

    &:insensitive {
      box-shadow:none;
    }
  }

  // Trough
  .trough {

    &:active,
    &:checked {
      background-color: $selected_bg_color;

      &:backdrop {
        background-color: $backdrop_selected_bg_color;
      }
    }

  }
}