message: PanelHeader: light theme is supported and documented
diff: /**
  * Header transition
  */
  +.View--animated .PanelHeader--ios {
  + transition: background-color .3s var(--ios-easing);
  + }
  +
  .PanelHeader--ios .PanelHeader__in--prev,
  .PanelHeader--ios .PanelHeader__in--next {
  overflow: hidden;
  