When creating an extension bundle for PlatformUI, you have to set up the configuration handling so that you can inject your own JavaScript module and/or CSS files in the JavaScript Application. This is described in the Setup the configuration tutorial page and this ends in changing the bundle extension class to have something like: https://github.com/ezsystems/ExtendingPlatformUIConferenceBundle/compare/1_bundle...2_configuration#diff-c9df54632274512f42e0bbf90ab80c52R21

This process is a bit cumbersome while this can and should be (semi-)automatic or at least simplified.