Changing button alignment to center on mobile? [closed]

I have used the following code and added it to Additional CSS under theme customization:

@media only screen and (max-width: 768px) {
.iphonebutton {
    text-align: center !important;
    padding: 0px !important;
    margin: 0px !important;
    display: block !important; } }

It is not changing the alignment for mobile to center. Not sure what I am doing wrong.