gnome-shell-dark-theme/gnome-shell-sass/widgets/_switches.scss

/* Switches */

$switch_width: 46px;
$switch_handle_size: 20px;

.toggle-switch {
  width: $switch_width;
  border-radius: $forced_circular_radius;
  transition-duration: 100ms;
  color: $fg_color;

  @if $contrast == 'high' {
    background: transparentize(if($variant == 'light', black, white), .7);

    &:hover {
      background: transparentize(if($variant == 'light', black, white), .6);
    }
  } @else {
    background: transparentize(if($variant == 'light', black, white), .85);

    &:hover {
      background: transparentize(if($variant == 'light', black, white), .8);
    }
  }

  StIcon {
    icon-size: $base_icon_size;
  }

  .handle {
    margin: 3px;
    width: $switch_handle_size;
    height: $switch_handle_size;
    border-radius: $forced_circular_radius;
    background: if($variant == 'light', white, mix(white, $bg_color, 80%));
    box-shadow: 0 2px 4px transparentize(black, .8);
    transition-duration: 100ms;
  }

  &:checked {
    background: -st-accent-color;
    color: -st-accent-fg-color;

    &:hover {
      background-color: st-lighten(-st-accent-color, 5%);
      color: st-lighten(-st-accent-fg-color, 5%);
    }

    .handle {
      background: white;
    }
  }
}