I tried to imitate demo content as much as I could, but I couldn’t change the header image as I wanted. So, uninstall and reinstall everything you need for troubleshoothing and demo content works. I already have a backup. If you can install the demo for me and check that everything is well, I´ll feel so fine.
If the Demo content works for you then why do you want me to install it again?
I don´t know if demo should be installed in “jevelin” or “jevelin-child” or both of them. Anyway I use to work with the child-theme.
Content any WordPress site is theme independent so you can use any theme that you want but child theme is recommended.