Не устанавливается цвет панели

Вариант 1:

Author: Smike

Source: https://forum.sources.ru

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, XPMan;

type
  TForm1 = class(TForm)
    Panel1: TPanel;
    XPManifest: TXPManifest;
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

uses
  Themes;

type
  TMyPanel = class(TPanel)
  public
    constructor Create(AOwner: TComponent); override;
  end;

{ TMyPanel }

constructor TMyPanel.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);

  if ThemeServices.ThemesEnabled then
    ControlStyle := ControlStyle - [csParentBackground] + [csOpaque];
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  R: TRect;
begin
  with TMyPanel.Create(Self) do
  begin
    Parent := Self;
    Color := clGreen;
    R := Panel1.BoundsRect;
    R.Left := R.Left + 300;
    R.Right := R.Right + 300;
    BoundsRect := R;
  end;
end;

end.

Вариант 2:

Автор: Krid

Source: https://forum.sources.ru

Можно отключать стили XP и для отдельных контролов (темы должны быть включены и манифест лежать на форме):

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, StdCtrls, ComCtrls, XPMan;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
    Button5: TButton;
    XPManifest1: TXPManifest;
    procedure FormCreate(Sender: TObject);
  private
    procedure Unload2Themes(var M:TMSG); message WM_USER+1;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function SetWindowTheme(hwnd: HWND; pszSubAppName: LPCWSTR; 
                        pszSubIdList: LPCWSTR): HRESULT; stdcall;  external 'uxtheme.dll';


procedure TForm1.Unload2Themes(var M: TMSG);
begin
  SetWindowTheme(Button4.Handle, ' ', ' ');
  SetWindowTheme(Button5.Handle, ' ', ' ');
  SetWindowTheme(Form1.Handle, ' ', ' ');
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
 PostMessage(Handle,WM_USER+1,0,0)
end;

end.