Either you use <ui:style> to get an implicit ClientBundle and CssResource (you can define an extended interface, other source CSS files to use, and inject the resulting CssResource into a @UiField), and it'll be automatically ensureInjected().
Or you use an existing ClientBundle and CssResource (with existing source CSS files) that you reference through <ui:with> (either GWT.create()d and possibly injected into a @UiField, or taken from a @UiField(provided=true), and you have to call ensureInjected() yourself (as with any other object used through <ui:with>)