lib/css.js: Use the path relative to lib/ to get the CSS file
When MCS starts, service-runner sets the CWD to the repo's root
directory, so relative paths will be relative to that path. However,
things change between source and deploy repos: to get to
private/base.css in the dpeloy repo, we need the relative path
src/private/base.css. In order to be able to satisfy all cases, make
the file path relative to the directory of lib/css.js rather than the
process' current CWD.