style.css

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

.container {
    width: 100%;
    min-height: 100vh;
    background: linear-gradient(-135deg, #23af32, #da23f3);
    padding: 10px;
}

.todo-app {
    width: 100%;
    max-width: 540px;
    background-color: #fff;
    margin: 100px auto 20px;
    padding: 40px 30px 70px;
    border-radius: 10px;
    -webkit-border-radius: 10px;
    -moz-border-radius: 10px;
    -ms-border-radius: 10px;
    -o-border-radius: 10px;
}

@media (min-width: 400px) and (max-width: 768px) {
    .todo-app {
        margin: 50% auto 20px;

    }

    ul li {
        justify-content: end;
        flex-direction: column;
        border-bottom: 1px solid #000;
    }
}

.todo-app h2 {
    margin-bottom: 20px;
}

.row {
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-radius: 50px;
    background-color: #eee;
    padding-left: 20px;
    margin-bottom: 25px;
    -webkit-border-radius: 50px;
    -moz-border-radius: 50px;
    -ms-border-radius: 50px;
    -o-border-radius: 50px;
}

.row input {
    flex: 1;
    outline: none;
    border: none;
    background: transparent;
    padding: 10px;
}

.row button {
    border: none;
    background-color: #555897;
    padding: 10px 20px;
    border-radius: 50px;
    cursor: pointer;
    color: white;
    font-weight: bold;
    font-size: 16px;
}

ul li {
    display: flex;
    align-items: center;
    justify-content: end;
    flex-direction: column;
    border-bottom: 1px solid #000;
    list-style: none;
    user-select: none;
    font-size: 18px;
    padding: 10px;
    cursor: pointer;
    position: relative;
}

ul li::before {
    content: '\f111';
    font-family: "Font Awesome 5 Free";
    font-weight: 900;
    color: white;
    border-radius: 50%;
    border: 1px solid #252b28;
    position: absolute;
    left: 0;
    top: 50%;
    font-size: 19px;
}

ul li.checked {
    text-decoration: line-through;
    color: #aaa;
    border-bottom-color: #aaa;
}

ul li.checked::before {
    content: '\f058';
    font-family: "Font Awesome 5 Free";
    font-weight: 900;
    color: #555897;
    border: none;
}

ul li span {
    display: flex;
    align-items: center;
    margin-top: 10px;

}

ul li i {
    display: flex;
    align-items: end;
}

ul li i::after {
    content: '\f068';
    font-family: "Font Awesome 5 Free";
    font-weight: 900;
    color: #555897;
    position: absolute;
    right: 0;
    top: 50%;
    font-size: 20px;
}