Apparently the backend doesn't set the MIME type header properly which new Chrome versions seem to dislike and prevent the style sheets from loading:
Refused to apply style from 'http://localhost:8888/nengo/51150/static/main.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
Apparently the backend doesn't set the MIME type header properly which new Chrome versions seem to dislike and prevent the style sheets from loading: