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,
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