We just updated to Confluence 5.7.4 and the sidebar is still shown in the old style (see screenshot) and not the "new" sidebar since 5.0.
We had a test server the last few weeks and there we had the new sidebar. We didn't change any of the settings from the test to the real server.
In addition to what Eduardo have suggested you, we can see that Confluence is kindly different from the default. For that, can you also validate if you are using any external add-ons to cover the theme? For example, the Brikit Theme, that you can also disable in the Manage Add-ons session.
PS: If all pages are like that, it's worth to check also for the *Look&Feel* session for Confluence Administrator Console.
Hope it helps you!!
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