I followed the guide (http://www.concrete5.org/documentation/how-tos/designers/modify-a-packaged-theme/) to fork a packaged theme and it has been working fine. I updated to the latest version of Concrete5 and noticed that the main CSS was not being applied to the site.

If I clear the cache, it appears to apply the CSS on the next time the page is loaded but if the home page is refreshed, the main.css is not loading correctly from the cache. If I try to load main.css directly, a blank document is returned, which would appear to indicate that the file is not being cached correctly.

The original theme works fine.

Any ideas?