Twitter has a group of beta users that it uses for testing out changes to its app and service on Android. These beta users have seen some rather rough changes (including some that made the app unusable) over the past few years, but sometimes these users get to see some UI changes that might hit the official app down the road. And this latest change is a minor UI tweak, but it makes the app look a whole lot different. Twitter is rolling out, at least to beta users via a server-side update, a more rounded interface. Profile pictures are now rounded, so are buttons like the "edit profile" button on user profile pages.
On top of seeing rounded icons, Twitter is also changing some of the other icons and turning them into silhouettes. For example, the icons in the slide-out drawer on the left, including the profile, lists, moments, etc, are all sihouettes, as are the icons in the new Tweet window that shows you Periscope live, pictures and video icons. Now the actual pictures and videos are not rounded, but the icons to switch to your camera are. They are in blue, while the icons in the slide-out drawer are in gray still. It's a bit of an interesting look, but it does look nice.
It's important to note here that these changes may not make it to the stable build of Twitter. The company has long added features to the beta version, only to remove them down the road, for one reason or another. This is a way for the company to test out changes, new features and such, without rolling it out to the millions of people using Twitter on Android. So while it is possible for some to see this change in the future at some point, it's also very likely that it may not come to the Twitter app. Now those that aren't in the beta program, you're a bit out of luck right now, as the beta program is closed, and it isn't letting anyone new in just yet. Which is unfortunate, but Twitter doesn't want millions of people in their beta test.