I used HTML5, CSS with custom properties and a mobile-first approach

To be honest I am not a CSS pro either but looking at your code I would say to keep your code as concise and easy to read as possible. You can take a look at the way I did it (even though it also might have some problems in it.)

    height: 100%;
}

html {
    display: table;
    margin: auto;
}

body {
    display: table-cell;
    vertical-align: middle;
    margin: 0;
    padding: 0;
    background-color: #d6e2f0;
    background-size: cover;
}

img {
    width: 300px;
    border-radius: 15px;
    margin-top: 15px;
}

h2 {
    padding-left: 25px;
    padding-right: 25px;
    text-align: center;
    font-family: 'Outfit';

}

.mainDiv {
    background-color: white;
    width: 330px;
    height: 500px;
    text-align: center;
    /* margin-top: 100px; */
    /* margin-left: 50px; */
    border-radius: 15px;
}

p {
    font-family: 'Outfit';
    color: #7b879d;
    padding-left: 40px;
    padding-right: 40px;
    font-size: 15px;
    text-align: center;
} ```

Please mark as helpful is this helped.