Use relative URLs for better preview when testing locally

I found that testing locally is difficult when having an absolute URL for the stylesheet.

Therefore I change this to a relative URL. The same of other occurrences in the file that should work with relative URLs as well. If someone puts an absolute URL there (for example the default URL), Hugo will still output the absolute URL if the host name is different from the hostname of the site.

Please let me know there have been reasons to have this absolute.

As this is only for testing locally, there is no need to hurry for a release.