<div class="Contenido">
        <asp:Panel ID="pnlForm" runat="server">
            <div class="Login form-horizontal">
                <div class="Barra">Entrar al sistema</div>
                <div class="form-group">
                    <label class="control-label col-xs-12 col-sm-4 col-md-4" for="<%= txtUser.ClientID%>">Usuario:</label>
                    <div class="col-xs-12 col-sm-7 col-md-7"><asp:TextBox ID="txtUser" runat="server" CssClass="form-control"></asp:TextBox></div>
                </div>
                <div class="form-group">
                    <label class="control-label col-xs-12 col-sm-4 col-md-4" for="<%= txtPwd.ClientID%>">Contraseña:</label>
                    <div class="col-xs-12 col-sm-7 col-md-7"><asp:TextBox ID="txtPwd" TextMode="Password" runat="server" CssClass="form-control"></asp:TextBox></div>
                </div>
                <div class="form-group">
                    <div class="col-sm-offset-3 col-sm-2">
                        <asp:Button ID="cmdAceptar" runat="server" Text="Aceptar" class="btn btn-sm btn-danger" />
                    </div>
                    <div class="col-sm-5">
                        <a class="LinkMail" href="RecuperarClave.aspx" style="line-height: 30px;">Olvidé mi Contraseña</a>
                    </div>
                </div>
                <div class="form-group">
                    <div class="col-sm-12 text-center">
                        <asp:label ID="lblError" ForeColor="Red" runat="server"></asp:label>
                    </div>
                </div>
            </div>
        </asp:Panel>
        <asp:Panel ID="pnlOut" runat="server" >
            <asp:label ID="lblOut" ForeColor="Red" runat="server"></asp:label>
        </asp:Panel>
    </div>

.Contenido
 {  width: 100%;
margin-left: auto ;
margin-right: auto ;

}

.Login {
    background-color: rgba(128, 128, 128, 0.35);
    width: 400px;
    text-align: center;
    margin: 50px auto 0;
    border-radius: 4px;
}