styles.css

* {
    box-sizing: border-box;
}
body {
    margin: 0;
    background-image: url("images/Background.jpg");
    background-size: cover;
    font-family: 'Open Sans', sans-serif;
}
.container {
    display: flex;
    height: 100vh;
    align-items: center;
    justify-content: center;
}

.not-found {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
}
.x {
    background-color: black;
    border-radius: 50%;
    width: 150px;
    height: 150px;
    display: flex;
    align-items: center;
    justify-content: center;
    position: relative;
}
.slash-r {
    width: 15px;
    background-color: #f980a9;
    height: 100px;
    transform: rotate(315deg);
}
.slash-l {
    width: 15px;
    background-color: #f980a9;
    height: 100px;
    transform: rotate(45deg);
    position: absolute;
   
}

.x::before {
    content: "4";
    position: absolute;
    left: -100%;
    font-family: 'Open Sans';
    font-size: 208px;
    color: white;
}
.x::after {
    content: "4";
    position: absolute;
    right: -100%;
    font-family: 'Open Sans';
    font-size: 208px;
    color: white;
}
.text {
    text-align: center;
    margin-top:50px;
    line-height: 2;
}
.text p {
    color: white;
   
}
.text p:nth-of-type(2) {
    color: #fedd0b;
}
form {
    width: 100%;
}
form input {
    border: 1px solid white;
    width: 600px;
    padding: 20px;
    background-color: transparent;
    caret-color: white;
    color: white;
  
}
form input:focus {
    outline: none;
}
form input::placeholder {
    color: white;
    opacity: 0.8;
}
form button {
    position: absolute;
    padding: 20px 40px;
    background-color: black;
    color: white;
    cursor: pointer;
    margin-left: 10px;    
}
@media (max-width:877px) {
    .not-found {
        width:50%;
    }
    form input {
        width: 100%;
    }
    form button {
        position: relative;
        margin-left: 0;
        margin-top: 15px;
    }
    .x {
        width: calc(150px / 2);
        height: calc(150px / 2);
    }
    .slash-l {
        height: calc(100px / 2);
        width: calc(15px / 2);
    }
    .slash-r {
        height: calc(100px / 2);
        width: calc(15px / 2);
    }
    .x::after {
        font-size: 104px;
    }
    .x::before {
        font-size: 104px;
    }
}
@media (max-width:474px) {
    .text p {
        font-size: 15px;
    }
}