I need to change the class of the OK button, by default it is dynamic and has been set to 'md-primary'.
$mdDialog.show(
$mdDialog.alert()
.parent(angular.element(document.querySelector('#popupContainer')))
.title('This is an alert title')
.textContent('You can specify some description text in here.')
.ariaLabel('Alert Dialog Demo')
.ok('Got it!') // trocar a cor desse botão
);