Add wxSystemAppearance to check for dark mode under macOS

Provide a way to retrieve the name of the current system appearance
(mostly for diagnostic purposes) and check if it uses predominantly dark
colours.

Currently this class has a non-trivial (but still very simple)
implementation under macOS only and simply checks whether the default
text colour is brighter than the default background colour under the
other platforms, but other platform-specific implementations could be
added later.

Also update the drawing sample "system colours" page to show the system
appearance as well.

include/wx/settings.h

@@ -162,6 +162,39 @@ enum wxSystemScreenType
     wxSYS_SCREEN_DESKTOP    //   >= 800x600
 };
 
+// ----------------------------------------------------------------------------
+// wxSystemAppearance: describes the global appearance used for the UI
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_CORE wxSystemAppearance
+{
+public:
+    // Return the name if available or empty string otherwise.
+    wxString GetName() const;
+
+    // Return true if the current system there is explicitly recognized as
+    // being a dark theme or if the default window background is dark.
+    bool IsDark() const;
+
+    // Return true if the background is darker than foreground. This is used by
+    // IsDark() if there is no platform-specific way to determine whether a
+    // dark mode is being used.
+    bool IsUsingDarkBackground() const;
+
+private:
+    friend class wxSystemSettingsNative;
+
+    // Ctor is private, even though it's trivial, because objects of this type
+    // are only supposed to be created by wxSystemSettingsNative.
+    wxSystemAppearance() { }
+
+    // Currently this class doesn't have any internal state because the only
+    // available implementation doesn't need it. If we do need it later, we
+    // could add some "wxSystemAppearanceImpl* const m_impl" here, which we'd
+    // forward our public functions to (we'd also need to add the copy ctor and
+    // dtor to clone/free it).
+};
+
 // ----------------------------------------------------------------------------
 // wxSystemSettingsNative: defines the API for wxSystemSettings class
 // ----------------------------------------------------------------------------
@@ -185,6 +218,9 @@ public:
     // get a system-dependent metric
     static int GetMetric(wxSystemMetric index, wxWindow * win = NULL);
 
+    // get the object describing the current system appearance
+    static wxSystemAppearance GetAppearance();
+
     // return true if the port has certain feature
     static bool HasFeature(wxSystemFeature index);
 };