Full _custom.scss

decoration {
  transition: $shadow_transition;
  border-radius: $wm_radius;
  box-shadow: 0 0 3px 1px $wm_shadow,
              0 0 8px 1px $wm_shadow,
              0 0 10px 1px $wm_shadow,
              0 0 16px 1px $wm_shadow,
              0 0 0 1px $wm_shadow,
              $wm_outline;

  // this is used for the resize cursor area
  margin: 6px;

  &:backdrop {
    // the transparent shadow here is to enforce that the shadow extents don't
    // change when we go to backdrop, to prevent jumping windows
    box-shadow: 0 0 3px 1px $wm_shadow_inactive,
                0 0 8px 1px $wm_shadow_inactive,
                0 0 10px 1px $wm_shadow_inactive,
                0 0 16px 1px $wm_shadow_inactive,
                0 0 0 1px $wm_shadow_inactive,
                $wm_outline_inactive;
  }

  .maximized &,
  .fullscreen &,
  .tiled &,
  .tiled-top &,
  .tiled-right &,
  .tiled-bottom &,
  .tiled-left & {
    border-radius: $maximized_radius;
    box-shadow: 0 3px 3px 0 $wm_shadow, $wm_outline;
  }

  .fullscreen & { border-radius: 0; }

  .popup & {
    box-shadow: none;
    border-radius: $wm_radius;
  }

  // server-side decorations as used by mutter
  .ssd & {
    margin: -5px;
    border-radius: $wm_radius $wm_radius 0 0;
    box-shadow: 0 0 5px 0 $wm_shadow, $wm_outline;

    &.maximized { border-radius: $maximized_radius $maximized_radius 0 0; }
  }

  .csd.popup & {
    border-radius: $mn_radius;
    box-shadow: 0 5px 8px rgba(0, 0, 0, 0.15), 0 8px 15px rgba(0, 0, 0, 0.08), $wm_outline;
  }

  tooltip.csd & {
    border-radius: $wm_radius;
    box-shadow: 0 1px 3px 0 rgba(0, 0, 0, 0.15);
  }

  messagedialog.csd & {
    border-radius: $wm_radius;
  }

  .solid-csd & {
    border: 1px solid $header_border;
    border-radius: 0;
    margin: 0;
    background-color: $header_bg;
    box-shadow: none;
  }
}