When i trie to add a menu in the Adapt authoring tool, an error message : Unrecognized plugin type for package adapt-iceCream popped up in the screen. can you tell me how to solve this problem and add a new plugin in the Adapt authoring tool.
You're getting that error because that theme is not authoring tool compatible.
To get it upgraded you can either log a ticket in the github repository to see if the theme's author will make the necessary changes - or you can try making the changes yourself.
As with anything open source it's good idea to check how recently it was updated before making a decision as to whether you use that code or not. For anything in github this is easy to do by browsing the github repository.