Var
sbx,sby:Integer;
p:TPanel;

p:=TPanel.Create(PageControl.Owner);
p.BevelOuter:=bvNone;
p.Parent:=PageControl.Parent;
p.BoundsRect:=PageControl.BoundsRect;
PageControl.Parent:=p;
sbx:=GetSystemMetrics(SM_CXEDGE)+GetSystemMetrics(SM_CXBORDER);
sby:=GetSystemMetrics(SM_CYEDGE)+GetSystemMetrics(SM_CYBORDER);
PageControl.SetBounds(-sbx,-sby,p.Width+sbx*2,p.Height+sby*2);

 TAdvPageControl = class(TCustomTabControl)
 private
   FPages: TList;
   FActivePage: TAdvTabSheet;
   FNewDockSheet: TAdvTabSheet;
   FUndockingPage: TAdvTabSheet;

   FTabMargin: TTabMargin;

  . . .

   procedure SetTabSplitLine(Value: Boolean);
   procedure SetRoundEdges(Value: Boolean);

   procedure SetTabMargins;

   procedure WMEraseBkgnd(var Message: TWmEraseBkgnd); message WM_ERASEBKGND;
   procedure WMPaint(var Message: TWMPaint); message WM_PAINT;

   . . .

TPageControl.Style := tsFlatButtons

 TXWinControl = class (TWinControl);
 TForm1 = class(TForm)
   PageControl1: TPageControl;
   TabSheet1: TTabSheet;
   TabSheet2: TTabSheet;
   Button1: TButton;
   Button2: TButton;
   TabSheet3: TTabSheet;
   TabSheet4: TTabSheet;
   procedure Button1Click(Sender: TObject);
 private
   procedure CustomDraw(Msg: TMessage);
   { Private declarations }
 public
   WndProc: tWndMethod;
   procedure OnPageWndProc(var Msg: TMessage);
   { Public declarations }
 end;
procedure TForm1.Button1Click(Sender: TObject);
begin
TXWinControl(PageControl1).Color := clRed;
WndProc := PageControl1.WindowProc;
PageControl1.WindowProc := OnPageWndProc;
TXWinControl(PageControl1).RecreateWnd;
PageControl1.Repaint;
end;

procedure TForm1.CustomDraw(Msg: TMessage);
var
 i: integer;
begin
   for i := 0 to PageControl1.PageCount-1 do
   begin
     PageControl1.Canvas.Rectangle(PageControl1.TabRect(i));
     PageControl1.Pages[i].Repaint;
   end;
   Msg.Result := 1;
end;

procedure TForm1.OnPageWndProc(var Msg: TMessage);
begin
 if (Msg.Msg = WM_NCPAINT ) or (Msg.Msg = WM_PAINT )then
 begin
   CustomDraw(Msg);
 end else
   WndProc(Msg);
end;