I didn't try for long time, but you may @here/harp-map-theme hosted directly from unpkg.com, try this:

https://unpkg.com/@here/harp-map-theme@0.28.0/resources/berlin_tilezen_effects_outlines.json

Same url with browser for available themes: https://unpkg.com/browse/@here/harp-map-theme@0.28.0

At least, unpkg shouldn't emit CORS failures.