研究一下FBrush，它是从TWinControl才有的属性（可能是因为需要句柄）

TWinControl = class(TControl)
private
  FBrush: TBrush;
end;

constructor TWinControl.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FObjectInstance := Classes.MakeObjectInstance(MainWndProc);
  FBrush := TBrush.Create;
  FBrush.Color := FColor;
  FParentCtl3D := True;
  FTabOrder := -1;
end;

procedure TWinControl.DefaultHandler(var Message);
begin
  if FHandle <> 0 then
  begin
    with TMessage(Message) do
    begin
      if (Msg = WM_CONTEXTMENU) and (Parent <> nil) then
      begin
        Result := Parent.Perform(Msg, WParam, LParam);
        if Result <> 0 then Exit;
      end;
      case Msg of
        WM_CTLCOLORMSGBOX..WM_CTLCOLORSTATIC:
          Result := SendMessage(LParam, CN_BASE + Msg, WParam, LParam);
        CN_CTLCOLORMSGBOX..CN_CTLCOLORSTATIC:
          begin
            SetTextColor(WParam, ColorToRGB(FFont.Color));
            SetBkColor(WParam, ColorToRGB(FBrush.Color));
            Result := FBrush.Handle; // 使用不算，还要返回
          end;
      else
        if Msg = RM_GetObjectInstance then
          Result := Integer(Self)
        else
        begin
          if Msg <> WM_PAINT then
          Result := CallWindowProc(FDefWndProc, FHandle, Msg, WParam, LParam);
        end;
      end;
      if Msg = WM_SETTEXT then
        SendDockNotification(Msg, WParam, LParam);
    end;
  end
  else
    inherited DefaultHandler(Message);
end;

procedure TWinControl.CMColorChanged(var Message: TMessage);
begin
  inherited;
  FBrush.Color := FColor; // 改变刷子的颜色
  NotifyControls(CM_PARENTCOLORCHANGED);
end;

procedure TWinControl.WMNCPaint(var Message: TMessage);
const
  InnerStyles: array[TBevelCut] of Integer = (0, BDR_SUNKENINNER, BDR_RAISEDINNER, 0);
  OuterStyles: array[TBevelCut] of Integer = (0, BDR_SUNKENOUTER, BDR_RAISEDOUTER, 0);
  EdgeStyles: array[TBevelKind] of Integer = (0, 0, BF_SOFT, BF_FLAT);
  Ctl3DStyles: array[Boolean] of Integer = (BF_MONO, 0);
var
  DC: HDC;
  RC, RW, SaveRW: TRect;
  EdgeSize: Integer;
  WinStyle: Longint;
begin
  { Get window DC that is clipped to the non-client area }
  if (BevelKind <> bkNone) or (BorderWidth > 0) then
  begin
    DC := GetWindowDC(Handle);
    try
      Windows.GetClientRect(Handle, RC);
      GetWindowRect(Handle, RW);
      MapWindowPoints(0, Handle, RW, 2);
      OffsetRect(RC, -RW.Left, -RW.Top);
      ExcludeClipRect(DC, RC.Left, RC.Top, RC.Right, RC.Bottom);
      { Draw borders in non-client area }
      SaveRW := RW;
      InflateRect(RC, BorderWidth, BorderWidth);
      RW := RC;
      if BevelKind <> bkNone then
      begin
        EdgeSize := 0;
        if BevelInner <> bvNone then Inc(EdgeSize, BevelWidth);
        if BevelOuter <> bvNone then Inc(EdgeSize, BevelWidth);
        with RW do
        begin
          WinStyle := GetWindowLong(Handle, GWL_STYLE);
          if beLeft in BevelEdges then Dec(Left, EdgeSize);
          if beTop in BevelEdges then Dec(Top, EdgeSize);
          if beRight in BevelEdges then Inc(Right, EdgeSize);
          if (WinStyle and WS_VSCROLL) <> 0 then Inc(Right, GetSystemMetrics(SM_CYVSCROLL));
          if beBottom in BevelEdges then Inc(Bottom, EdgeSize);
          if (WinStyle and WS_HSCROLL) <> 0 then Inc(Bottom, GetSystemMetrics(SM_CXHSCROLL));
        end;
        DrawEdge(DC, RW, InnerStyles[BevelInner] or OuterStyles[BevelOuter],
          Byte(BevelEdges) or EdgeStyles[BevelKind] or Ctl3DStyles[Ctl3D] or BF_ADJUST);
      end;
      IntersectClipRect(DC, RW.Left, RW.Top, RW.Right, RW.Bottom);
      RW := SaveRW;
      { Erase parts not drawn }
      OffsetRect(RW, -RW.Left, -RW.Top);
      Windows.FillRect(DC, RW, Brush.Handle); // 使用刷子绘制非客户区
    finally
      ReleaseDC(Handle, DC);
    end;
  end;

  inherited;

  if ThemeServices.ThemesEnabled and (csNeedsBorderPaint in ControlStyle) then
    ThemeServices.PaintBorder(Self, False);
end;

procedure TCustomForm.WMIconEraseBkgnd(var Message: TWMIconEraseBkgnd);
begin
  if FormStyle = fsMDIChild then
  if (FormStyle = fsMDIChild) and not (csDesigning in ComponentState) then
    FillRect(Message.DC, ClientRect, Application.MainForm.Brush.Handle)
  else inherited;
end;

procedure TCustomForm.CMColorChanged(var Message: TMessage);
begin
  inherited;
  if FCanvas <> nil then FCanvas.Brush.Color := Color;
end;