Board Thread:Support Requests - Getting Technical/@comment-5751180-20131205210800

I would like to know how to customize the message you see when visiting a non-existent page. This is because the wiki I edit (UK Charts Archive Wiki) needs a more meaningful message, like "The page, chart or stats page does not currently exist. Make sure you have typed in the correct date if it is a chart week, or if it is a stats page, check it hasn't been created under a different name".

If it involves any CSS, please could you tell me exactly what to do, because in my previous experience CSS editing doesn't seem to do anything. 