fix: restore theme output refresh compatibility (#4378)

Author: cpsievert

NEWS.md

@@ -8,7 +8,9 @@
   output-info pipeline (image/plot sizing, hidden-state tracking, theme
   reporting) work automatically in any layout — including CSS-only show/hide,
   third-party tab components, and non-Bootstrap frameworks — without requiring
-  custom event hooks. (#3682)
+  custom event hooks. This also introduces a `shiny:themechange` event
+  for code that needs to trigger theme clientdata refreshes after changing
+  surrounding visual theme context. (#3682)
 
 # shiny 1.13.0