I'm trying to add the navigation sidebar as described in documentation but when I go to "Space Admin" and choose "Layout" from "Look and Feel" panel I get this message:
"You can not customise the site look and feel while a theme is active. You must first disable currently configured theme."

However I cannot see any option to disable currently selected theme.

I have following themes available: "Default Theme" (selected), "Left Navigation Theme", "Confluence Classic Theme", "Confluence Classic Theme (with old typography)", "Clickr Theme".

This issue is related to upgrading from older version of Confluence because when we installed new version and imported the same space I was able to edit layout.