style.css

* {
    margin: 0;
    padding: 0;
}

.nav-box {
    background-color: #333333;
    display: flex;
    flex-direction: row;
    height: 58px;
}

.nav-box>a {
    padding: 15px;
    padding-left: 25px;
    padding-right: 25px;
    color: white;
    text-decoration: none;
    font-size: 24px;
}

.active {
    background-color: #04AA6D !important;
}

.nav-box>a:hover {
    background-color: #000;
}

.main-box {
    display: flex;
    height: calc(100vh - 58px);
    justify-content: center;
    align-items: center;
}

.arrow-box {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 10vh;
    font-size: 24px;
    padding: 30px;
    cursor: pointer;
}

.arrow-box:hover {
    background-color: #e0e0e0;
}

.todo-box {
    margin-left: 10px;
    margin-right: 10px;
    width: 700px;
    font-size: 24px;
    display: flex;
    flex-direction: column;
    text-align: center;
    align-items: center;
    justify-content: center;
}

.todo-button {
    margin-top: 10px;
    height: 25px;
    width: 25px;
    background-color: rgb(250, 250, 250);
    border: 1px solid black;
    cursor: pointer;
}

.todo-button:hover {
    background-color: rgb(235, 235, 235);
}