- 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.