JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link href="https://fonts.googleapis.com/css?family=Jura" rel="stylesheet">
    <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.1.1/css/all.css" integrity="sha384-O8whS3fhG2OnA5Kas0Y9l3cfpmYjapjI0E4theH4iuMD+pLhbf6JI0jIMfYcK3yZ"
        crossorigin="anonymous">
    <link rel="stylesheet" type="text/css" href="css/style.css">
    <script src="js/jquery-3.3.1.min.js"></script>
    <title>To Do List</title>
</head>

<body>
    <div class="container">
        <div class="title">
            <span>to-do list</span>
            <span id="addSign">
                <i class="fas fa-plus"></i>
            </span>
        </div>
        <div class="list">
            <ul>
                <li id="addField">
                    <input type="text" placeholder="Add New Todo" id="inputField">
                </li>
                <!-- <li class="whBckg">
                    <div class='deleteSign' style="display: none"><i class="fas fa-trash-alt"></i></div>
                    <span class='list-item '>To jest test</span>
                </li> -->
            </ul>
        </div>
    </div>

    <script src="js/index.js"></script>
</body>

</html>

CSS

body {
    background: linear-gradient(to right, rgb(0, 217, 255), rgb(255, 238, 0)) no-repeat fixed;
    font-family: 'Jura', sans-serif;
}

.container {
    margin: 150px auto;
    width: 25%;
    min-width: 400px;
    height: auto;
    /* border: 2px solid black; */
}

.title {
    width: 100%;
    height: 100%;
    padding-top: 10px;
    padding-bottom: 10px;
    /* border: 2px solid pink; */
    background-color: rgb(48, 142, 250);


}

.title span {
    font-size: 28px;
    font-weight: bold;
    color: white;
    text-transform: uppercase;
    padding-left: 5%;
}

#addSign {
    font-size: 28px;
    font-weight: bold;
    color: white;
    text-transform: uppercase;
    float:right;
    padding-right: 5%;
}

.list {
    width: 100%;
}

ul {
    margin: 0;
    padding: 0;
    list-style: none;
}

ul li {
    background: rgb(235, 235, 235);
    /* padding: 10px; */
}

ul li input{
    display: block;
    width: 100%;
    padding: 10px;
    border: 0;
    margin: 0;
    font-size: 16px;
    background: none;
    color: rgb(0, 119, 231);
}

ul li input:focus {
    outline: none;
}

.whBckg {
    background-color: white;
}

.grBckg {
    background-color: rgb(235, 235, 235);
}

.fa-trash-alt {
    color: white;
}

.deleteSign {
    padding: 10px;
    display: inline-flex;
    background-color: red;
}

.list-item {
    display: inline-block;
    padding: 10px;
}

.marked {
    text-decoration: line-through;
    color: gray;
}

JavaScript

$('#addSign').on('click', function() {
    $('#addField').slideToggle();
})

$('ul').on('click', 'li', function(){
    $(this).children('.list-item').toggleClass('marked');
});

$('li').on('mouseenter', 'li', function() {
    $(this).children('.deleteSign').animate({width:'toggle', paddingLeft: 'toggle', paddingRight: 'toggle'},350);
});

$('ul').on('mouseleave', 'li', function(){
    $(this).children('.deleteSign').animate({width:'toggle', paddingLeft: 'toggle', paddingRight: 'toggle'},350);
})

 $('ul').on('click', 'li', function() {
    $(this).children('.deleteSign').closest('li').remove();
})

$('#inputField').on('keypress', function(e) {
    if(e.charCode === 13) {
        const value = ($(this).val());
        $(this).val('');
        const markup = `<li class="whBckg">
        <div class='deleteSign' style="display: none"><i class="fas fa-trash-alt"></i></div>
        <span class='list-item'>${value}</span>
        </li>`;
        $('ul').append(markup);

    }
})