I can verify that as of today (Feb 25, 2016), this problem is still happening. A <span> element with a 120px left shim is still pushed in, and needs to be dealt with if you're doing any custom styling. To Corry's point about specificity in CSS selectors, getting down to the specific <span> is probably your best bet in solving this quickly.
In our case, it's declared this way in our form's custom CSS:
.mktoButtonRow span[class="mktoButtonWrap mktoSimple"] {
margin:auto!important;
}
The end result is that the margin-left attribute that's hard-coded gets ignored. At least from there you can increase the predictability in how things will style. And obviously, your margins may vary depending on your need.
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