style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    color: #777;
}

.header {
    height: 100vh;
    position: relative
}

.text-box {
    position: absolute;
    top: 50px;
    left: 50%;
    transform: translateX(-50%);
    text-align: center;
}

.text-404 {
    color: red;
    font-size: 60px;
}

.text-404-p {
    color: #000;
    font-size: 30px;
    margin-top: 20px;
}

.text-404-p span {
    display: block;
}

.text-404-p span:last-child {
    margin-top: 25px;
}

.img-box {
    position: absolute;
    width: 90%;
    display: flex;
    flex-wrap: wrap;
    justify-content: space-around;
    margin: 0 auto;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -25%);
}

.img-box-img {
    flex-basis: 30%;
    text-align: center;
    margin-bottom: 20px;
    color: white;
    text-align: center;
}

.img-box-img img {
    height: 250px;
}

.btn-fir {
    margin-top: 20px;
}

.btn-fir a {
    padding: 10px 20px;
    font-size: 20px;
    border-radius: 12px;
    border: 1px solid green;
    text-decoration: none;
    color: #000000;
}

.btn-fir button:focus {
    outline: none;
}

.footer-text {
    position: absolute;
    bottom: 20px;
    margin-left: 50px;
}

.footer-text h1 a {
    text-decoration: none;
    color: #000;
    border: 2px solid green;
    padding: 0 13px;
    border-radius: 12px;
}