main.css

body {
    margin: 0px;
    padding: 0px;
    background-color: turquoise;
}

#box {
    height: 200px;
    width: 1000px;
    background-color: #2D2D2D;
    border-radius: 5px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

#main {
    height: 400px;
    width: 430px;
    background-color: white;
    border-radius: 5px;
    position: absolute;
    top: 50%;
    left: 70%;
    transform: translate(-50%, -50%);
    z-index: 99;
}

#loginform,
#signupform {
    position: absolute;
    top: 50%;
    left: 70%;
    transform: translate(-50%, -50%);
    z-index: 999;
}

#signupform {
    top: 45%;
    left: 75%;
    visibility: hidden;
}

#loginform h1,
#signupform h1 {
    font-family: arial;
    font-size: 25px;
    color: turquoise;
}

#loginform input,
#signupform input {
    height: 40px;
    width: 300px;
    border: 0px;
    outline: none;
    border-bottom: 1px solid black;
    margin: 5px;
}

#loginform button,
#signupform button {
    height: 35px;
    width: 130px;
    background-color: turquoise;
    font-family: monospace;
    font-size: 16px;
    color: white;
    border: none;
    outline: none;
    border-radius: 5px;
    margin-top: 30px;
    margin-left: 175px;
}

#login_btn,
#signup_btn {
    height: 35px;
    width: 120px;
    background-color: transparent;
    color: white;
    border: 1px solid white;
    border-radius: 5px;
    outline: none;
    position: absolute;
    left: 75%;
    top: 65%;
    transform: translate(-50%, -50%);
    transition: all .5s;
}

#signup_btn {
    left: 25%;
}

#login_btn:hover,
#signup_btn:hover {
    background-color: white;
    color: #2d2d2d;
    cursor: pointer;
}

#login_msg,
#signup_msg {
    font-family: arial;
    font-size: 25px;
    color: white;
    position: absolute;
    top: 35%;
    left: 75%;
    transform: translate(-50%, -50%);
    z-index: 1;
}

#signup_msg {
    left: 25%;
}