Improve samples appearance in dark mode

- caret: Use the system color for window background instead of
  hard-coded white.

- image: Use hard-coded black for the text drawing since all other
  drawings on the image canvas use hard-coded colours too. The default
  text colour was barely readable in dark mode.

- joytest: Use wxCYAN_PEN instead of wxBLACK_PEN to ensure the line is
  visible both in light and dark colour system mode.

- ownerdrw: Use such background colour that there is good contrast
  between text and background both in light and dark colour system mode.

- popup: Use such background colour for the pop up so that there is a
  good contrast between text and background both in light and dark
  colour system mode.

Closes #23773.

samples/caret/caret.cpp

@@ -306,7 +306,7 @@ MyCanvas::MyCanvas( wxWindow *parent )
 {
     m_text = nullptr;
 
-    SetBackgroundColour(*wxWHITE);
+    SetBackgroundColour(wxSystemSettings::GetColour(wxSYS_COLOUR_WINDOW));
 
     SetFontSize(12);
 

samples/image/canvas.cpp

@@ -401,6 +401,7 @@ void MyCanvas::OnPaint( wxPaintEvent &WXUNUSED(event) )
     wxPaintDC dc( this );
     PrepareDC( dc );
 
+    dc.SetTextForeground(*wxBLACK);
     dc.DrawText( "Loaded image", 30, 10 );
     if (my_square.IsOk())
         dc.DrawBitmap( my_square, 30, 30 );

samples/joytest/joytest.cpp

@@ -193,7 +193,7 @@ void MyCanvas::OnPaint(wxPaintEvent& WXUNUSED(evt))
     else if (m_stick->GetButtonState(3))
         dc.SetPen(*wxYELLOW_PEN);
     else
-        dc.SetPen(*wxBLACK_PEN);
+        dc.SetPen(*wxCYAN_PEN);
 
     dc.DrawLine(FromDIP(m_pos), FromDIP(pt));
 

samples/ownerdrw/ownerdrw.cpp

@@ -297,7 +297,7 @@ OwnerDrawnFrame::OwnerDrawnFrame(wxFrame *frame, const wxString& title,
     for ( ui = 0; ui < WXSIZEOF(aszChoices); ui += 2 )
     {
 #if defined(__WXMSW__) && !defined(__WXUNIVERSAL__)
-        m_pListBox->GetItem(ui)->SetBackgroundColour(wxColor(200, 200, 200));
+        m_pListBox->GetItem(ui)->SetBackgroundColour(*wxBLUE);
 #endif
     }
 
@@ -345,6 +345,10 @@ OwnerDrawnFrame::OwnerDrawnFrame(wxFrame *frame, const wxString& title,
         {
             pListBox->GetItem(ui)->SetBackgroundColour(wxColor(0, 0, 0));
         }
+        else if ( ui == 5 ) // black on dark grey is barely visible
+        {
+            pListBox->GetItem(ui)->SetBackgroundColour(*wxYELLOW);
+        }
     }
 
 #else

samples/popup/popup.cpp

@@ -109,8 +109,10 @@ SimpleTransientPopup::SimpleTransientPopup( wxWindow *parent, bool scrolled )
                                               wxBORDER_NONE |
                                               wxPU_CONTAINS_CONTROLS )
 {
+    wxColour colour = wxSystemSettings::SelectLightDark(*wxLIGHT_GREY, wxColour(90, 90, 90));
+
     m_panel = new wxScrolledWindow( this, wxID_ANY );
-    m_panel->SetBackgroundColour( *wxLIGHT_GREY );
+    m_panel->SetBackgroundColour(colour);
 
     // Keep this code to verify if mouse events work, they're required if
     // you're making a control like a combobox where the items are highlighted
@@ -214,7 +216,7 @@ void SimpleTransientPopup::OnMouse(wxMouseEvent &event)
     wxRect rect(m_mouseText->GetRect());
     rect.SetX(-100000);
     rect.SetWidth(1000000);
-    wxColour colour(*wxLIGHT_GREY);
+    wxColour colour = wxSystemSettings::SelectLightDark(*wxLIGHT_GREY, wxColour(90, 90, 90));
 
     if (rect.Contains(event.GetPosition()))
     {