message: Implement TranslucentButton on GTK+ 3
  Use truly translucent button via custom CSS styling.
diff: @@ -404,6 +404,7 @@ TranslucentButton::TranslucentButton(wxWindow *parent, wxWindowID winid, const w
  TranslucentButton::TranslucentButton(wxWindow *parent, wxWindowID winid, const wxString& label)
  {
  wxButton::Create(parent, winid, label);
  +
  #ifdef __WXMSW__
  if (IsWindows10OrGreater())
  {
  @@ -416,6 +417,23 @@ TranslucentButton::TranslucentButton(wxWindow *parent, wxWindowID winid, const w
  });
  }
  #endif
  +
  +#ifdef __WXGTK3__
  + GTKApplyCssStyle(R"(
  + * {
  + background-image: none;
  + background-color: rgba(255,255,255,0.5);
  + color: rgba(0,0,0,0.7);
  + text-shadow: none;
  + border-color: rgba(0,0,0,0.3);
  + border-image: none;
  + border-radius: 2;
  + }
  + *:hover {
  + background-color: rgba(255,255,255,0.7);
  + }
  + )");
  +#endif
  }
  #endif // !__WXOSX__
  