style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: Verdana, Geneva, Tahoma, sans-serif;
}

body {
    height: 100vh;
    background-image: linear-gradient(#a514df, #d43bc0);
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
}

.container {
    width: 85vw;
    max-width: 700px;
    height: 80vh;
    border-radius: 50px;
    box-shadow: 2px 2px 20px rgba(34, 24, 24, 0.911);
    background-color: rgb(234, 193, 253);
    display: flex;
    flex-direction: column;
    align-items: center;
}

header {
    font-size: 2rem;
    color: rgb(41, 41, 41);
    text-shadow: 3px 5px 10px black;
    text-align: center;
    padding: 1rem;
}

.todo-add {
    width: 90%;
    display: flex;
    margin-top: 1rem;
    font-size: 0.5rem;
}
#todo-input {
    width: 80%;
    margin-right: 10px;
    border-color: #fff;
    border-radius: 10px;
    outline: none;
    padding: 0.5rem;
}

#todo-button {
    width: 20%;
    padding: 0.7rem;
    background-color: #5d20bf;
    color: #fff;
    font-size: 1rem;
    font-weight: bold;
    border-radius: 10px;
    border: none;
}

#todo-button:hover {
    opacity: 0.3;
}

.todos {
    width: 90%;
    margin-top: 2rem;
    overflow-x: hidden;
    overflow-y: auto;
}

.todos ul {
    list-style: none;
}

ul li {
    background-color: #eee;
    color: rgb(41, 41, 41);
    font-weight: bold;
    margin-bottom: 1rem;
    display: flex;
    align-items: center;
}

ul li:nth-child(even) {
    background: #dde6f5;
}

li i {
    padding: 1rem;
    color: #ea0a0a;
    cursor: pointer;
}

li i:hover {
    opacity: 0.3;
}

li p {
    flex: auto;
    padding: 0.2rem;
}

li.checked > .fa-check {
    color: #00e676;
}

li.checked > p {
    color: rgb(41, 41, 41);
    text-decoration: line-through;
    font-weight: normal;
}

@media screen and (max-width: 576px) {
    li p {
        font-size: 0.8rem;
    }
}