How can I set the color of a docked form in a tab sheet to the correct color?

When I dock a TForm to a TTabSheet the form has a gray background color. The tab sheet on the other hand has a white background color.

It gets more complicated when theming is disabled (e.g. classic Windows theme).

With the current code the grey form has a white border which is pretty ugly.

So how do I set the form background color to the tab sheet background color? In case this doesn't work: How do I set the tab sheet background color to the forms background color?

It should work with runtime themes enabled and runtime themes disabled.

program Project1;

uses
  Graphics,
  Controls,
  Forms,
  ComCtrls;

{$R *.res}

var
  Main        : TForm;
  Sub         : TForm;
  PageControl : TPageControl;
  TabSheet    : TTabSheet;
begin
  Application.Initialize;
  Application.MainFormOnTaskbar := True;
  Application.CreateForm(TForm, Main);
  Application.CreateForm(TForm, Sub);
  PageControl        := TPageControl.Create(Main);
  PageControl.Parent := Main;
  PageControl.Align  := alClient;
  TabSheet             := TTabSheet.Create(Main);
  TabSheet.PageControl := PageControl;
  Sub.Dock(TabSheet, TabSheet.ClientRect);
  Sub.Align := alClient;
  Sub.Show;
  // Sub.Color := clWhite; // TabSheet.Color;
  Application.Run;
end.

I found this workaround

type
  TWinControlAccess = class(TWinControl)
  end;

procedure TMainForm.CreateEmbedded(FormClass: TFormClass; Parent: TWinControl);
var
  form: TForm;
begin
  form := FormClass.Create(Self);
  form.Align := alClient;
  form.BorderIcons := [];
  form.BorderStyle := bsNone;
  form.Parent := Parent;
  TWinControlAccess(form).ParentBackground := True; // <<<-
  form.Show;
end;