style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Quicksand', sans-serif;
}

.container {
    width: 100%;
    min-height: 100vh;
    background: linear-gradient(#fadee7, #ec2d76, #ecf573);
    padding: 10px;
}

.todo-app {
    width: 100%;
    max-width: 600px;
    background: #fff;
    margin: 100px auto 20px;
    padding: 40px 30px 70px;
    border-radius: 20px;
}

.todo-app h2 {
    color: rgb(15, 15, 15);
    display: flex;
    align-items: center;
    margin-bottom: 2%;

}

.row {
    display: flex;
    align-items: center;
    justify-content: space-between;
    background: #eddef0;
    border-radius: 30px;
    padding-left: 20px;
    margin-bottom: 25px;
}

input {
    flex: 4;
    border: none;
    outline: none;
    background: transparent;
    padding: 10px;
    font-weight: 20px;
}

button {
    border: none;
    outline: none;
    background: #000000;
    color: #fff;
    padding: 15px 30px;
    font-size: 15px;
    cursor: pointer;
    border-radius: 40px;
}

ul li {
    list-style: none;
    font-size: 17px;
    padding: 12px 8px 12px 50px;
    user-select: none;
    cursor: pointer;
    position: relative;
}

ul li::before {
    content: '';
    position: absolute;
    height: 30px;
    width: 28px;
    border-radius: 50px;
    background-image: url(images/unchecked.png);
    background-size: cover;
    background-position: center;
    top: 15px;
    left: 8px;

}
ul li.checked
{
    color: #555;
    text-decoration: line-through;

}
ul li.checked::before{
    background-image: url(images/checked2.png);
    background-size: cover;
    background-position: center;
    top: 15px;
    left: 8px;
}
ul li span{
    position: absolute;
    height: 40px;
    width: 40px;
    top: 5px;
    right: 0;
    font-size: 22px;
    color: #555;
    line-height: 40px;
    text-align: center;
}
ul li span:hover{
    background-color: #b6a9a9;
}