Ivan_009,
Код:

[Setup]
AppName=My Application
AppVersion=1.5
DefaultDirName={pf}\My Application
[Code]
var
Test : boolean;
function InitializeSetup : boolean;
begin
Test := MsgBox('Использовать стандартное оформление?', mbInformation, MB_YESNO) = idYes;
Result := True;
end;
procedure InitializeWizard;
begin
case Test of
True: begin
with WizardForm do begin
ClientWidth := ScaleX(400);
ClientHeight := ScaleX(400);
Color := clRed;
end;
end;
False: begin
with WizardForm do begin
ClientWidth := ScaleX(500);
ClientHeight := ScaleX(200);
Color := clLime;
end;
end;
end;
end;