Change style of cell if group collapses?

I haven't figured out how to do this with styles yet but I feel as though I have made significant progress and I want to share it. Please try:

t1 = True;

Notebook[{Cell[
    CellGroupData[{
      Cell["Parent", "Section", 
        Background -> Dynamic[If[t1, LightRed, LightBlue]]], 
      Cell["Fee", "Subsection"],
      Cell["Fi", "Subsection"],
      Cell["Fo", "Subsection"],
      Cell["Fum", "Subsection"]
    }, Dynamic[t1]]
]}] // NotebookPut

Here is a style sheet solution:

Notebook[
    {
    Cell[StyleData[StyleDefinitions->"Default.nb"]],
    Cell[StyleData["Section"],
        ShowGroupOpener->True,
        CellContext->Cell,
        CellDynamicExpression :> With[{cell = NotebookRead[EvaluationCell[]]},
            NotebookWrite[EvaluationCell[], Cell[""], All];
            NotebookWrite[
                EvaluationNotebook[],
                Cell @ CellGroupData[
                    {
                    Replace[cell,
                        Cell[b_, "Section", ___] :>
                        Cell[b, "Section",
                            CellDynamicExpression:>None,
                            Background->Dynamic[If[open, White, LightBlue]]
                        ]
                    ]
                    },
                    Dynamic[open]
                ]
            ];
            FrontEndTokenExecute["MovePrevious"]
        ]
    ]       
    },
    Saveable->False,WindowSize->{808,689},WindowMargins->{{Automatic,143},{40,Automatic}},
    FrontEndVersion->"10.3 for Mac OS X x86 (32-bit, 64-bit Kernel) (December 10, 2015)",
    StyleDefinitions->"PrivateStylesheetFormatting.nb"
];
NotebookPut @ %;

The first NotebookWrite is a hack so that the second NotebookWrite actually does what I want, which is to create a CellGroupData object with a Dynamic in it's second argument.