ThemePrefs.cpp has fewer dependencies...

... Send an event via the app that projects listen to for theme update; don't
push it to projects directly.

This frees three files from dependency cycles.

src/Project.cpp

@@ -165,6 +165,7 @@ scroll information.  It also has some status flags.
 #include "commands/CommandTargets.h"
 #include "commands/CommandType.h"
 
+#include "prefs/ThemePrefs.h"
 #include "prefs/QualityPrefs.h"
 #include "prefs/TracksPrefs.h"
 
@@ -1425,6 +1426,8 @@ AudacityProject::AudacityProject(wxWindow * parent, wxWindowID id,
                      &AudacityProject::OnCapture,
                      this);
 
+   wxTheApp->Bind(EVT_THEME_CHANGE, &AudacityProject::OnThemeChange, this);
+
 #ifdef EXPERIMENTAL_DA2
    ClearBackground();// For wxGTK.
 #endif
@@ -1566,6 +1569,19 @@ void AudacityProject::OnCapture(wxCommandEvent& evt)
       mIsCapturing = false;
 }
 
+void AudacityProject::OnThemeChange(wxCommandEvent& evt)
+{
+   evt.Skip();
+   ApplyUpdatedTheme();
+   for( int ii = 0; ii < ToolBarCount; ++ii )
+   {
+      ToolBar *pToolBar = GetToolManager()->GetToolBar(ii);
+      if( pToolBar )
+         pToolBar->ReCreateButtons();
+   }
+   GetRulerPanel()->ReCreateButtons();
+}
+
 
 const std::shared_ptr<DirManager> &AudacityProject::GetDirManager()
 {

src/Project.h

@@ -541,6 +541,7 @@ public:
  private:
 
    void OnCapture(wxCommandEvent & evt);
+   void OnThemeChange(wxCommandEvent & evt);
    void InitialState();
 
  public:

src/prefs/ThemePrefs.cpp

@@ -31,13 +31,15 @@ Provides:
 #include "../Audacity.h"
 #include "ThemePrefs.h"
 
+#include <wx/app.h>
 #include <wx/wxprec.h>
 #include "../Prefs.h"
 #include "../Theme.h"
-#include "../Project.h"
 #include "../ShuttleGui.h"
 #include "../AColor.h"
 
+wxDEFINE_EVENT(EVT_THEME_CHANGE, wxCommandEvent);
+
 enum eThemePrefsIds {
    idLoadThemeCache=7000,
    idSaveThemeCache,
@@ -207,24 +209,12 @@ void ThemePrefs::OnSaveThemeAsCode(wxCommandEvent & WXUNUSED(event))
    theTheme.WriteImageDefs();// bonus - give them the Defs too.
 }
 
-#include "../AdornedRulerPanel.h"
-#include "../toolbars/ToolManager.h"
-
 void ThemePrefs::ApplyUpdatedImages()
 {
    AColor::ReInit();
 
-   for (size_t i = 0; i < gAudacityProjects.size(); i++) {
+   wxCommandEvent e{ EVT_THEME_CHANGE };
-      AudacityProject *p = gAudacityProjects[i].get();
+   wxTheApp->SafelyProcessEvent( e );
-      p->ApplyUpdatedTheme();
-      for( int ii = 0; ii < ToolBarCount; ++ii )
-      {
-         ToolBar *pToolBar = p->GetToolManager()->GetToolBar(ii);
-         if( pToolBar )
-            pToolBar->ReCreateButtons();
-      }
-      p->GetRulerPanel()->ReCreateButtons();
-   }
 }
 
 /// Update the preferences stored on disk.

src/prefs/ThemePrefs.h

@@ -15,11 +15,16 @@
 #define __AUDACITY_THEME_PREFS__
 
 #include <wx/defs.h>
+#include <wx/event.h> // to declare a custom event type
 
 #include "PrefsPanel.h"
 
 class ShuttleGui;
 
+// An event sent to the application when the user changes choice of theme
+wxDECLARE_EXPORTED_EVENT(AUDACITY_DLL_API,
+                         EVT_THEME_CHANGE, wxCommandEvent);
+
 #define THEME_PREFS_PLUGIN_SYMBOL ComponentInterfaceSymbol{ XO("Theme") }
 
 class ThemePrefs final : public PrefsPanel