Sometimes the generated CSS is buggy

Initially reported by softec.

Sometimes, the first request to a CSS file obtains a strange results. For example, Flamingo's style.css is generated without any color theme.

According to softec, it used to happen in the past with Colibri CSS file too, but it was not so disturbing since the other requests obtain the good output.

But the problem is that now LESS caches the output, so the error comes back until someone re-saves the skin or the color theme and so refreshes the cache.

Not easy to debug, I've never hit this locally, but it is frequent on staging server using Tomcat and MySQL. I cannot tell if it is related or not.