A theme handles qa_content. So, modifying qa-theme won't be good, especially if you've got many closed questions on a page.
You may either modify qa_content for existing "unanswered" page, or create a new page with similar name (not exactly "unanswered"). In the former case you should apply changes to the inner Q&A code, so it will be a bit harder to upgrade to next versions, and there may be some problems with other plugins.
Make your choice and go ask further :)