Youtube keeps track of your watch history and automatically marks videos that you have already watched. They however only mark recently seen videos, which is kind of disappointing. This addon fixes this issue and keeps track of your entire watch history, such that not only the recently seen videos are being marked.
Does this work with the old Youtube theme? I am afraid that only the new theme is supported. If you do not know which theme you are currently using then it is very likely the new one.
How can the persistence of the database be ensured? There is an export and import feature, which makes it easy to create a backup of the history. This backup can easily be archived such that you can ensure that you never lose your history.
How can I make sure that the database is complete? The automatic synchronization only considers the recent activity. You are thus encouraged to manually initiate a complete synchronization in the settings, which incorporates as much of your history as possible.