Fix invisible captions in high contrast mode

Don't use black text on dark background.

Closes #16186.

include/wx/aui/tabart.h

@@ -289,6 +289,8 @@ protected:
     wxFont m_normalFont;
     wxFont m_selectedFont;
     wxFont m_measuringFont;
+    wxColour m_normalTextColour;
+    wxColour m_selectedTextColour;
     wxPen m_normalBkPen;
     wxPen m_selectedBkPen;
     wxBrush m_normalBkBrush;