Suggesting the best theme is like asking for the best color, the best food or best hobby: It depends on your personal taste and preferences.
On Nothing phones, you can’t apply whole theme packages, you can only change the icon packs. Personally I have been using the official Nothing icon pack ever since I got my NP2 and have been satisfied with the way it looks.
You can look and search for more icon packs either by opening the play store or directly when trying to select an icon pack, the “Add icon pack” option gives you a shortcut to the play store.
