Set wxPG cell background colour as wxPGComboBox background colour

Default wxGenericComboCtrl background colour is obtained under wxGTK
with wxComboBox::GetClassDefaultAttributes(). For wxGTK 3 returned colour
is fully transparent (00000000) which can cause problems with
proper drawing of controls deriving from wxGenericComboCtrl, like
wxPGComboBox which in turn is used by wxEnumProperty, wxCursorProperty,
etc.
wxPGComboBox should have background with the same colour as the cell
so we can explicitly set background colour to avoid using default one.

Closes #17986.

src/propgrid/editors.cpp

@@ -1072,6 +1072,8 @@ wxWindow* wxPGChoiceEditor::CreateControlsBase( wxPropertyGrid* propGrid,
 #endif // !__WXOSX__
     cb->SetMargins(wxPG_XBEFORETEXT-1);
 
+    cb->SetBackgroundColour(propGrid->GetCellBackgroundColour());
+
     // Set hint text
     cb->SetHint(property->GetHintText());