居中引导程序组表单

Question

我对此很陌生。我有这段代码，我需要将所有内容居中。

<form name="input" action="@Url.Action("Login", "BackOffice")" method="post">
        <div class="form-group">
            <label class="control-label col-sm-2" for="nombre">Nombre de usuario: </label>
            <input required type="text" name="nombre" id="nombre" placeholder="Cccebo" class="form-control" />
        </div>
        <div class="form-group">
            <label class="control-label col-sm-2" for="contrasenia">Contraseña: </label>
            <input required type="password" name="contrasenia" id="contrasenia" placeholder="*********" class="form-control" />
        </div>
        @Html.ActionLink("¿Olvidaste tu contraseña? Hacé click acá!", "CambiarContrasenia", "BackOffice") <br />
        <input type="submit" name="Submit" value="Enviar" class="btn btn-info" style="margin-left: 405px" />
        <br />
        <b>@ViewBag.Mensaje</b>
    </form>

This is what it looks like