That is very odd.
Do you experience the same issue when you go to the Theme page in the demo?
What web browser are you using?
What error message(s) appear in your web server logs when you try to change the theme?
When you try to edit the custom field by clicking the Edit icon, nothing happens. This is a glitch with osTicket itself, it is not an osTicket Awesome issue. I have opened a report with the osTicket devs and hopefully they will have a fix soon.
In the meantime you can edit the ticket itself and then edit the contents of the custom field.