style.css

<style>
    body {
        background-color: #fff;
        color: #353535;
        font-family: sans-serif;
        padding: 2rem;
        max-width: 768px;
        margin: 0 auto;
    }

    #app {
        margin-left: 0 auto;
        margin-right: 0 auto;
        padding-left: 0;
        padding-right: 0;
    }

    #form {
        margin-top: 2rem;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    #fileExtention {
        flex-basis: 100%;
        max-width: 200px;
        margin-right: 0;
    }

    #submit {
        flex-basis: auto;
        font-weight: bold;
        background-color: #585858;
        border: none;
        padding: 1rem;
        margin-top: 2rem;
        transition: all 0.2s;
    }

    #submit:enabled {
        background-color: #5676e5;
        color: #fff;
        cursor: pointer;
    }

    #submit:disabled {
        background-color: #868e96;
        color: #fff;
        cursor: not-allowed;
    }

    #loader {
        opacity: 0;
        visibility: hidden;
        transition: all 1s;
    }

    #loader.hidden {
        opacity: 0;
        visibility: visible;
    }

    #backButton {
        margin-top: 2rem;
    }

    #backButtonLoader {
        opacity: 0;
    }
</style>