Probably because 'jumper.com' is not a valid site. This link:
Does not resolve to a valid web site. (Try it yourself.) There is nothing there.
This link works:
However, WordPress prefers to handle these errors internally, if it can. Read this article to find out how to do that and what to put in your .htaccess file:
I don't know about that. They have a completely different appearance to the main site, so any visitor who comes upon them and isn't web-savvy (i.e., 97.5% of your visitors) probably won't realize what they are looking at and will experience a minor freak-out. You always have to view design issues from a user's point of view, not your own.
This page is older, but also has good information:
(However, its method of creating a child theme is entirely out of date. We no longer use the "@import" method.)