Add `toolkit-theme' and 'toolkit-theme-set-functions'

* etc/NEWS: Document `toolkit-theme' and
'toolkit-theme-set-functions'
* lisp/frame.el: Add 'toolkit-theme-set-functions'
* src/frame.c: Add `toolkit-theme' Elisp variable
* src/gtkutil.c: (xg_update_dark_mode_for_all_displays): Set
`toolkit-theme' and call 'toolkit-theme-set-functions'
* src/w32fns.c: (w32_applytheme): Send WM_EMACS_SET_TOOLKIT_THEME
message to Lisp thread.
* src/w32term.h: Declare WM_EMACS_SET_TOOLKIT_THEME
* src/w32term.c: (w32_read_socket): Receive
WM_EMACS_SET_TOOLKIT_THEME message and set `toolkit-theme' and call
'toolkit-theme-set-functions'

etc/NEWS

@@ -3603,6 +3603,13 @@ Emacs when built with the pure GTK toolkit now respects desktop
 dark and light modes for drawing a GTK toolbar and widgets,
 automatically toggling between them.
 
+---
+** toolkit-theme-set-functions called when the toolkit theme is set for Emacs.
+When the theme is set on PGTK or MS-Windows builds,
+'toolkit-theme-set-functions' is called.  Result is stored in var
+'toolkit-theme' as either symbol 'dark' or 'light', but may be
+expanded to future toolkit-specific symbols in the future.
+
 
 * Changes in Emacs 31.1 on Non-Free Operating Systems
 

lisp/frame.el

@@ -1528,6 +1528,12 @@ the `background-mode' terminal parameter."
 ;;                         :background (face-attribute 'default :foreground))
 ;;     (frame-set-background-mode (selected-frame))))
 
+(defvar toolkit-theme-set-functions nil
+  "Abnormal hook run when the system theme is applied to Emacs.
+Functions on this hook are called with the theme name as a symbol:
+`light' or `dark'.  By the time the hook is called, `toolkit-theme' will
+already be set to one of these values as well.")
+
 
 ;;;; Frame configurations
 

src/frame.c

@@ -7618,4 +7618,17 @@ The default is \\+`inhibit' in NS builds and nil everywhere else.  */);
   Fprovide (Qmove_toolbar, Qnil);
 #endif /* !HAVE_EXT_TOOL_BAR || USE_GTK */
 #endif /* HAVE_WINDOW_SYSTEM */
+
+#if (defined(HAVE_PGTK) && defined(HAVE_GSETTINGS)) || defined (WINDOWSNT)
+  DEFVAR_LISP ("toolkit-theme", Vtoolkit_theme,
+               doc: /* The current toolkit theme.
+Either the symbol `light' or the symbol `dark', reflecting the system's
+current theme preference.  This variable is updated automatically when
+the system theme changes.
+
+This variable is only available on PGTK and MS-Windows builds.  */);
+  Vtoolkit_theme = Qnil;
+  DEFSYM (Qlight, "light");
+  DEFSYM (Qdark, "dark");
+#endif
 }

src/gtkutil.c

@@ -1475,6 +1475,16 @@ xg_update_dark_mode_for_all_displays (bool dark_mode_p)
 	 = gtk_settings_get_for_screen (screen);
        xg_set_gtk_theme_dark_mode (dark_mode_p, settings);
      }
+
+   Vtoolkit_theme = dark_mode_p ? Qdark : Qlight;
+   Lisp_Object hook = intern ("toolkit-theme-set-functions");
+   if (!NILP (Fboundp (hook)))
+     {
+       Lisp_Object args[2];
+       args[0] = hook;
+       args[1] = Vtoolkit_theme;
+       Frun_hook_with_args (2, args);
+     }
 }
 
 /* Set initial dark mode for a new frame (called during frame

src/w32fns.c

@@ -2440,6 +2440,8 @@ w32_applytheme (HWND hwnd)
 				    &w32_darkmode, sizeof (w32_darkmode));
 	}
     }
+  WPARAM dark_mode_p = w32_darkmode ? 1 : 0;
+  PostThreadMessage (dwMainThreadId, WM_EMACS_SET_TOOLKIT_THEME, dark_mode_p, 0);
 }
 
 static HWND
@@ -3645,6 +3647,7 @@ w32_name_of_message (UINT msg)
       M (WM_CHAR),
       M (WM_EMACS_DRAGOVER),
       M (WM_EMACS_DROP),
+      M (WM_EMACS_SET_TOOLKIT_THEME),
 #undef M
       { 0, 0 }
   };

src/w32term.c

@@ -6083,6 +6083,20 @@ w32_read_socket (struct terminal *terminal,
 	  check_visibility = 1;
 	  break;
 
+	case WM_EMACS_SET_TOOLKIT_THEME:
+	  {
+	    Vtoolkit_theme = msg.msg.wParam ? Qdark : Qlight;
+	    Lisp_Object hook = intern ("toolkit-theme-set-functions");
+	    if (!NILP (Fboundp (hook)))
+	      {
+		Lisp_Object args[2];
+		args[0] = hook;
+		args[1] = Vtoolkit_theme;
+		Frun_hook_with_args (2, args);
+	      }
+	  }
+	  break;
+
 #if HAVE_W32NOTIFY
 	case WM_EMACS_FILENOTIFY:
 	  f = w32_window_to_frame (dpyinfo, msg.msg.hwnd);

src/w32term.h

@@ -715,6 +715,7 @@ do { \
 #define WM_EMACS_DRAGOVER              (WM_EMACS_START + 27)
 #define WM_EMACS_DROP                  (WM_EMACS_START + 28)
 #define WM_EMACS_END                   (WM_EMACS_START + 29)
+#define WM_EMACS_SET_TOOLKIT_THEME     (WM_EMACS_START + 30)
 
 #define WND_FONTWIDTH_INDEX    (0)
 #define WND_LINEHEIGHT_INDEX   (4)