You are correct Jack. The style property does not add the styling to the input textbox but the surrounding div. This we have to change in a upcoming release. What I would suggest for the textbox is to add a class in your form and put this into your stylesheet. See example and screenshot below:
.default .custom-textbox label {
font-weight:bold;
font-size:20px;
}
If you want to center your button you can just use text-align: center; on the table cell since the link button is an inline element. See screenshot.
As a common rule in Mendix: if css does n't work, set it on the surrounding element.