The documentation theme was removed as of version 6.x as you highlighted and it was transitioned to the default theme. You can read the end of support notice here - https://confluence.atlassian.com/conf59/end-of-support-announcements-for-confluence-792499890.html and a transition guide here - https://confluence.atlassian.com/conf59/switch-to-the-default-theme-792498652.html.
Hope this helps,
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG