Did you read the javadocs for that method? I don't think it does what you think it does. Try addStyleName instead.
Is this in a CssResource in your app? If not, that class name will be obfuscated when it is used, and since your css isn't, you class won't apply. This CssResource belongs in a new appearance, extending from the Grid's default appearance.
Other options: get a reference to the scroller object and add an additional css class name that matches your css (something like grid.getView().getScroller()).