style.css

:root {
    --color1: #3177E1;
    --color1-dark: #2962b8;
    --color2: #3D3D3D;
}

* {
    margin: 0;
    padding: 0;
    font-family: 'Open Sans', arial, sans-serif;
}

h1 {
    font-family: museo-sans, Arial, Helvetica, sans-serif;
    font-weight: 700;
    font-size: 36px;
    text-align: center;
    color: white;
    margin-bottom: 20px;
}

label {
    font-family: museo-sans, Arial, Helvetica, sans-serif;
    font-weight: 300;
    font-size: 14px;
    color: white;
    margin-bottom: 5px;
}

input[type=text], input[type=number], select {
    height: 30px;
    border: 0px solid #FFFFFF;
    border-radius: 3px;
    padding-left: 10px;
}

input[type=submit] {
    background-color: var(--color1);
    font-family: museo-sans, Arial, Helvetica, sans-serif;
    font-weight: 700;
    font-size: 17px;
    text-align: center;
    color: white;
    border: 0;
    align-items: center;
    border-radius: 3px;
    width: 260px;
    height: 45px;
}

input[type=submit]:hover {
    background-color: var(--color1-dark);
    color: #cfcfcf;
}

.container {
    background-size: cover;
    height: 100vh;
    background: url(img/background.png) center center;
    align-items: center;
    justify-content: center;
    display: flex;
    flex-direction: column;
}

.content {
    background-color: var(--color2);
    padding: 35px 60px 35px 60px;
    box-shadow: 0px 15px 16px #0000002E;
    border-radius: 6px;
}

.row {
    display: flex;

}

.form-content {
    display: flex;
    justify-content: flex-start;
    flex-direction: column;
    text-align: start;
    padding: 10px;
}

.filename, .filesize {
    flex-grow: 20;
}

.fileext, .sizeunit {
    flex-grow: 1;
}

.submit {
    margin-top: 33px;
    display: flex;
    justify-content: center;
}