Thomas is obviously one of the main characters. His friends like Percy, James, and Gordon are also likely to be main characters as they are usually important in Thomas and Friends stories.
One main theme could be friendship. Thomas and his friends rely on each other, which shows the importance of friendship in their world. Another might be trust, as the title mentions 'trust Thomas'. They trust each other to do their jobs and help out on the railway.
Well, 'Thomas Christmas Party' is likely a story full of holiday cheer. In the world of Thomas, his stories usually involve his friends on the railway. There could be fun adventures, like getting ready for the Christmas party, decorating the station, and sharing presents. The 'Other Thomas Stories' might include tales of Thomas helping out other engines, or perhaps getting into some small mischief on the tracks.
Children can learn about friendship. Thomas has many friends on the railway, and they all help each other. Also, they can learn about the joy of giving, especially in the Christmas Party story where sharing gifts and having a celebration together is important.
It could be about Thomas making a new friend and other various tales. Maybe it involves the adventures that Thomas has with this new friend and how it changes his world or relationships.
Since Thomas is mentioned, he is surely one of the characters. As for the new friend, we don't know the name yet but that's another important character. There could also be other characters like Thomas's family or the people in his community depending on the nature of the story.