You can use ng-class
to add conditional classes.
HTML
<button id="button1" ng-click="alpha = true" ng-class="{alpha: alpha}">Button</button>
In your controller (to make sure the class is not shown by default)
$scope.alpha = false;
Now, when you click the button, the $scope.alpha variable is updated and ng-class will add the 'alpha' class to your button.