feat(windows): add 'Follow system theme' option for dark mode

On macOS, BambuStudio already auto-follows the system dark/light theme
via EVT_SYS_COLOUR_CHANGED. On Windows this handler was guarded with
#ifndef __WINDOWS__, so theme changes were silently ignored.

Changes:
- GUI_Utils.hpp: EVT_SYS_COLOUR_CHANGED now fires on Windows when
  'dark_mode_follow_system' is enabled; calls update_dark_config() and
  on_sys_color_changed() to refresh the UI
- GUI_App.cpp dark_mode(): if follow-system is enabled, query
  check_dark_mode() (system appearance) instead of the stored config;
  on startup, sync dark_color_mode config from system if follow-system
  is enabled
- MainFrame.cpp on_sys_color_changed(): call force_colors_update() and
  update_ui_from_settings() on all platforms (previously skipped on
  Windows), so a system-triggered theme change gets a full UI refresh
- Preferences.hpp/.cpp: add 'Follow system theme' checkbox below the
  manual 'Enable dark mode' toggle; when checked the manual toggle is
  disabled and the current system state is applied immediately

Resolves feature request #10800.

Author: BenJule

src/slic3r/GUI/GUI_App.cpp

@@ -3016,11 +3016,18 @@ bool GUI_App::on_init_inner()
     load_language(wxString(), true);
 #ifdef _MSW_DARK_MODE
 
-#ifndef __WINDOWS__
-    wxSystemAppearance app = wxSystemSettings::GetAppearance();
-    GUI::wxGetApp().app_config->set("dark_color_mode", app.IsDark() ? "1" : "0");
-    GUI::wxGetApp().app_config->save();
-#endif // __APPLE__
+    {
+        wxSystemAppearance app = wxSystemSettings::GetAppearance();
+#ifdef __WINDOWS__
+        if (app_config->get("dark_mode_follow_system") == "1") {
+            app_config->set("dark_color_mode", app.IsDark() ? "1" : "0");
+            app_config->save();
+        }
+#else
+        GUI::wxGetApp().app_config->set("dark_color_mode", app.IsDark() ? "1" : "0");
+        GUI::wxGetApp().app_config->save();
+#endif
+    }
 
 
     bool init_dark_color_mode = dark_mode();
@@ -3643,7 +3650,9 @@ bool GUI_App::dark_mode()
     // proper dark mode was first introduced.
     return wxPlatformInfo::Get().CheckOSVersion(10, 14) && mac_dark_mode();
 #else
-    return wxGetApp().app_config->get("dark_color_mode") == "1" ? true : check_dark_mode();
+    if (wxGetApp().app_config->get("dark_mode_follow_system") == "1")
+        return check_dark_mode();
+    return wxGetApp().app_config->get("dark_color_mode") == "1";
     //const unsigned luma = get_colour_approx_luma(wxSystemSettings::GetColour(wxSYS_COLOUR_WINDOW));
     //return luma < 128;
 #endif

src/slic3r/GUI/GUI_Utils.hpp

@@ -201,12 +201,17 @@ template<class P> class DPIAware : public P
 
         this->Bind(wxEVT_SYS_COLOUR_CHANGED, [this](wxSysColourChangedEvent& event)
         {
-#ifndef __WINDOWS__
+#ifdef __WINDOWS__
+            if (GUI::wxGetApp().app_config &&
+                GUI::wxGetApp().app_config->get("dark_mode_follow_system") == "1") {
                 update_dark_config();
                 on_sys_color_changed();
-                event.Skip();
-#endif // __WINDOWS__
-
+            }
+#else
+            update_dark_config();
+            on_sys_color_changed();
+#endif
+            event.Skip();
         });
 
         if (std::is_same<wxDialog, P>::value) {

src/slic3r/GUI/MainFrame.cpp

@@ -2611,10 +2611,8 @@ void MainFrame::on_sys_color_changed()
     // update label colors in respect to the system mode
     wxGetApp().init_label_colours();
 
-#ifndef __WINDOWS__
     wxGetApp().force_colors_update();
     wxGetApp().update_ui_from_settings();
-#endif //__APPLE__
 
 #ifdef __WXMSW__
     wxGetApp().UpdateDarkUI(m_tabpanel);

src/slic3r/GUI/Preferences.cpp

@@ -1452,7 +1452,43 @@ wxWindow* PreferencesDialog::create_general_page()
     //dark mode
 #ifdef _WIN32
     auto title_darkmode = create_item_title(_L("Dark Mode"), page, _L("Dark Mode"));
-    auto item_darkmode = create_item_darkmode_checkbox(_L("Enable dark mode"), page,_L("Enable dark mode"), 50, "dark_color_mode");
+    auto item_darkmode = create_item_darkmode_checkbox(_L("Enable dark mode"), page, _L("Enable dark mode"), 50, "dark_color_mode");
+
+    // "Follow system theme" checkbox
+    auto sizer_follow = new wxBoxSizer(wxHORIZONTAL);
+    sizer_follow->Add(0, 0, 0, wxEXPAND | wxLEFT, 23);
+    m_dark_mode_follow_system_checkbox = new ::CheckBox(page);
+    m_dark_mode_follow_system_checkbox->SetValue(app_config->get("dark_mode_follow_system") == "1");
+    sizer_follow->Add(m_dark_mode_follow_system_checkbox, 0, wxALIGN_CENTER, 0);
+    sizer_follow->Add(0, 0, 0, wxEXPAND | wxLEFT, 8);
+    auto follow_label = new wxStaticText(page, wxID_ANY, _L("Follow system theme"), wxDefaultPosition, wxDefaultSize, 0);
+    follow_label->SetForegroundColour(DESIGN_GRAY900_COLOR);
+    follow_label->SetFont(::Label::Body_13);
+    sizer_follow->Add(follow_label, 0, wxALIGN_CENTER | wxALL, 3);
+    m_dark_mode_follow_system_checkbox->SetToolTip(_L("Automatically switch dark mode to match the Windows system theme."));
+    if (app_config->get("dark_mode_follow_system") == "1")
+        m_dark_mode_ckeckbox->Enable(false);
+    m_dark_mode_follow_system_checkbox->Bind(wxEVT_TOGGLEBUTTON, [this](wxCommandEvent& e) {
+        bool follow = m_dark_mode_follow_system_checkbox->GetValue();
+        app_config->set("dark_mode_follow_system", follow ? "1" : "0");
+        app_config->save();
+        m_dark_mode_ckeckbox->Enable(!follow);
+        if (follow) {
+            bool sys_dark = wxSystemSettings::GetAppearance().IsDark();
+            app_config->set("dark_color_mode", sys_dark ? "1" : "0");
+            m_dark_mode_ckeckbox->SetValue(sys_dark);
+        }
+        wxGetApp().Update_dark_mode_flag();
+#ifdef _MSW_DARK_MODE
+        wxGetApp().force_colors_update();
+        wxGetApp().update_ui_from_settings();
+        set_dark_mode();
+#endif
+        SimpleEvent evt = SimpleEvent(EVT_GLCANVAS_COLOR_MODE_CHANGED);
+        wxPostEvent(wxGetApp().plater(), evt);
+        e.Skip();
+    });
+    auto item_darkmode_follow = sizer_follow;
 #endif
 
 #if 0
@@ -1561,6 +1597,7 @@ wxWindow* PreferencesDialog::create_general_page()
 #ifdef _WIN32
     sizer_page->Add(title_darkmode, 0, wxTOP | wxEXPAND, FromDIP(20));
     sizer_page->Add(item_darkmode, 0, wxEXPAND, FromDIP(3));
+    sizer_page->Add(item_darkmode_follow, 0, wxEXPAND, FromDIP(3));
 #endif
 
 #if 0

src/slic3r/GUI/Preferences.hpp

@@ -95,7 +95,8 @@ class PreferencesDialog : public DPIDialog
     // debug mode
     ::CheckBox * m_developer_mode_ckeckbox   = {nullptr};
     ::CheckBox * m_internal_developer_mode_ckeckbox = {nullptr};
-    ::CheckBox * m_dark_mode_ckeckbox        = {nullptr};
+    ::CheckBox * m_dark_mode_ckeckbox               = {nullptr};
+    ::CheckBox * m_dark_mode_follow_system_checkbox = {nullptr};
     ::TextInput *m_backup_interval_textinput = {nullptr};
 
     wxString m_developer_mode_def;