style.css

* {
    margin: 0;padding: 0;
    box-sizing: border-box;
}
html, body {
    width: 100%; height: 100%;
    font-family: 'sans-serif';
    font-size: 18px;
    color: #555;
}
.header {
    text-align: center;
    margin-top: 20px;
}
.app {
    width: 100%;
    max-width: 400px;
    margin: 20px auto 0;
    padding: 10px 0;
    overflow: hidden;
    box-shadow: 5px 5px 5px grey;
    border-radius: 5px;
    box-shadow: rgba(0, 0, 0, 0.35) 0px 5px 15px;
}

/* form         */
/* ------------ */
.task-form {
    margin: 10px 5px 0 5px;
}
.task-form__input {
    padding: 5px;
    width: 270px;
    margin-right: 10px;
    color: #444;
    font-size: inherit;
    outline: none;
    border: 2px solid #999;
    border-radius: 5px;
}
.input-error {
    border: 1px solid red;
}
.task-form__button {
    padding: 5px;
    border-radius: 5px;
    font-size: inherit;
}

/* task         */
/* ------------ */
.task {
    padding-left: 30px;
    padding-top: 20px;
}
.task__element {
    padding: 5px;
}
.task__wrapper {
    display: flex;
    justify-content: space-between;
}
.task__wrapper--name {
    color: grey;
    cursor: pointer;
}
.task__completed {
    text-decoration: line-through;
    color: red;
}
.task__button--button {
    background-color: #ff481f69;
    padding: 5px 10px;
    border-radius: 5px;
    cursor: pointer;
}