The problem with moving them out of static/luna is that there's luna specific paths in the css file. In your case, the best solution would be to leave core.css as is, and just change include_common_head.html to use <%config.db_static_url%>/<%t%>.css and <%config.db_static_url%>/luna/core.css. That way you'll get the changes in the core.css file on upgrades, and your custom changes won't be as confusing.

Adrian