@media screen and (max-width: 660px) {
    .container {
        flex-direction: column;
        height: 100vh;
        width: 100vw;
    }
    

    .big {
        border: none;
        background-color: rgba(0, 0, 0, 0.4);
    }
    
    
    /*alapok*/
    #login_side, #register_side {
        position: relative;
        width: 100vw;
        height: 50vh;
    
        transition: width 1s ease-in-out;
    }

    #login_small h2, #register_small h2 {
        margin-top: 20px;
    }

    #vissza {
        opacity: 1;
        transition: opacity 0.5s ease-in-out;
    }
    
}