SOURCE

$('#it').focus(function () {
    $('#it').addClass('searchFocus');
})

$('#it').blur(function () {
    if (!$('#it').val()) {
        $('#it').removeClass('searchFocus');
    }
})

<form>
    <input type="button" value="搜索">
    <input id="it" type="text" placeholder="Search ..." class="search">
</form>

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 90vh;
}

form {
    width: 500px;
    display: flex;
}

input[type='button'] {
    margin-right: 10px;
    outline: none;
    border-radius: 5px;
    border: 1px solid #ddd;
    font-weight: 600;
}

.search {
    border: 1px solid #ccc;
    border-radius: 5px;
    outline: none;
    background-color: skyblue;
    transition: all 0.2s ease-in-out;
}

input::-webkit-input-placeholder {
    font-weight: 800;
}

input.whiteFont::-webkit-input-placeholder {
    color: #ccc;
}

.searchFocus {
    color: #fff;
    background-color: #333744;
    flex-grow: 1;
}