header input {
    border: 2px solid green;
    width: 15%;
    height: 3.315508021390374331550802139037vh;
    margin-left: 4%;
    position: relative;
}

header input::before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    right: 100%;
    bottom: 0;
    background-color: blue;
    width: 10%;
}