- if there is no preceding / character, we look for the file within all the appropriate theme directories.
- otherwise the file should have a preceding /, which means use this path relative to the hubzilla basedir
- files beginning with // are considered to be schemeless URLs.
Therefore 'foo.css' means find the best theme-able representation of foo.css.
'/library/foo.css' means only use the version of foo.css that is in /library.