Changing button alignment to center on mobile?

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.