Make a CSS responsive

Question

I am developing a system with Asp.Net Mvc using bootstrap, but I want to give a different "face" to my login screen, I created a css but I would like to make it responsive, does anyone know how to do that? follows the CSS below: more specifically the "loginbox" style (which is where my login form is)

    body {
    margin: 0;
    padding: 0;
    background: url('../images/orage.png');
    background-size: cover;
    background-position: center;
    font-family: sans-serif;
}

    .loginbox {
        width: 320px;
        height: 420px;
        background: #000;
        color: #fff;
        top: 50%;
        left: 50%;
        position: absolute;
        transform: translate(-50%, -50%);
        box-sizing: border-box;
        padding: 70px 30px;
        max-width: 100%;

    }



    .avatar {
        width: 100px;
        height: 100px;
        border-radius: 50%;
        position: absolute;
        top: -50px;
        left: calc(50% - 50px);
    }

    h1 {
        margin: 0;
        padding: 0 0 20px;
        text-align: center;
        font-size: 22px;
    }

    .login p {
        margin: 0;
        padding: 0;
        font-weight: bold;
    }

    .loginbox input {
        width: 100%;
        margin-bottom: 20px;
    }

    .footer {
        position: inherit;
        bottom: 0;
        width: 100%;
    }

Follows the HTML

@using (Html.BeginForm("Login", "Account", new { ReturnUrl = ViewBag.ReturnUrl }, FormMethod.Post, new { @class = "form-horizontal", role = "form"  }))
{
    @Html.AntiForgeryToken()
    ;

    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <br>
    <br>
    <div class="loginbox">

        <img src="~/images/avatar.png" class="avatar" />
        <h1>Login Here</h1>
        @Html.EditorFor(model => model.Email, new { htmlAttributes = new { @class = "form-control input-lg", placeholder = "E-mail", autofocus = true } })
        @Html.ValidationMessageFor(model => model.Email, "", new { @class = "text-danger" })


        @Html.EditorFor(model => model.Password, new { htmlAttributes = new { @class = "form-control input-lg", placeholder = "Senha" } })
        @Html.ValidationMessageFor(model => model.Password, "", new { @class = "text-danger" })
        <input type="submit" value="Acessar" class="btn primary btn-lg" />
        <br/>
        <a href="#">Lost your password? </a>
        <a href="#"> Don't have an acount? </a>

    </div>
}

Screenshot of the login screen