The updated window has a cleaner design with either a dark theme or a light theme based on your preferences. In the screen below, the darker theme is depicted, with the original window on the left and the new window on the right.
The composition window now includes support for adding GIFs to a tweet, a new addition to TweetDeck, and there is also an option to create a poll. A new emoji picker has been added, and support for image descriptions and tags is new.
TweetDeck provides the ability to switch between the new composing window and the old one, so those who prefer the original interface can stick to it at the moment.
Guess what? You are lucky! Starting today, we are trying a new way to tweeting, now with the ability to add GIFs, threads, polls, emoji and image tagging via TweetDeck 🎉🙌🎊
̵1; TweetDeck (@TweetDeck) May 16 2019
The new features are part of a server side update, so you don't need to download a new version of the app to get the updated composing window.
TweetDeck for Mac can be downloaded from the Mac App Store for free. [Direct Link]