.mainPage {
    display: flex; /* missing! */
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 60px 20px 20px 20px;
    min-height: 100svh; /* safer than 100vh on iOS Safari */
    text-align: center;
    box-sizing: border-box;
}

.mainPage h1{
    margin: 0;
}

.mainPage h4{
    font-weight: normal;
    margin: 0;
}

button {
    display: block;
    padding: 12px;
    width: 20vw;
    margin: 30px 0 15px 0;
    border-radius: 25px;
    transition: 0.5s ease;
    cursor: pointer;
    font-size: 20px;
    color: black;
    background-color: white;
    border: 2px solid black; /* make sure border is visible */
    box-sizing: border-box;
}

button:hover {
    color: white;
    background-color: black;
    border: solid black;
    transform: scale(105%);
}

@media (max-width: 768px) {
    button {
        width: 80%;
    }
}