Pure.css

body 
{
    background: linear-gradient(rgba(41, 39, 39, 0.8), rgba(53, 75, 83, 0.8)),url(img/Login.gif);

    height: 100vh;

    background-size: cover;

    background-position: center;  
}

.searchBox {
    position: absolute;
    top: 10%;
    left: 50%;
    transform:  translate(-50%,50%);
    background: #2f3640;
    height: 40px;
    border-radius: 40px;
    padding: 10px;

}

.searchBox:hover > .searchInput {
    width: 240px;
    padding: 0 6px;
}

.searchBox:hover > .searchButton {
  background: rgb(26, 25, 25);
  color : #2f3640;
}

.searchButton {
    color: white;
    float: right;
    width: 40px;
    height: 40px;
    border-radius: 50%;
    background: #2f3640;
    display: flex;
    justify-content: center;
    align-items: center;
    transition: 0.4s;
}

.searchInput {
    border:none;
    background: none;
    outline:none;
    float:left;
    padding: 0;
    color: white;
    font-size: 16px;
    transition: 0.4s;
    line-height: 40px;
    width: 0px;

}

@media screen and (max-width: 620px) {
.searchBox:hover > .searchInput {
    width: 150px;
    padding: 0 6px;
}
}