.my-custom-css {
    color: red;
    font-size: 16px;
    padding: 10px;
}

@media (max-width: 768px) {
    .my-custom-css {
        font-size: 14px;
    }
    
    .responsive-element {
        width: 100%;
        height: auto;
    }
}

.header-nav {
    background-color: #333;
    margin: 20px 0;
}

@media (max-width: 768px) {
    .header-nav {
        background-color: #222;
    }
    
}