As a common rule in Mendix: if css does n't work, set it on the surrounding element.
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.