/**
* Custom Style for Oolson goes here IF REALLY NEEDED
* You should use the SASS structure instead in scss/*
**/
.gform_footer {
    text-align: center; /* Center-aligns the content of the footer where the button is */
}

body .gform_wrapper .gform_footer input[type=submit] {
    margin: 0 auto; /* Auto margins for left and right will center the button */
    display: inline-block; /* Or 'block' if 'inline-block' does not work */
}