See this Amazon press release for details
"The light theme is much appreciated, giving users a choice between white or black borders in the library, home, and store sections of the app. There are a few other visual changes astute observers may pick up on, including the addition of Amazon’s custom Bookerly font for book titles and an added emphasis on book covers with larger display images. Beyond the fresh coat of paint, Amazon is adding a new “one-tap access” feature that floats your current book at the bottom of the app in a miniaturized rectangle that follows you around the app. That way, you can easily jump back in at any time. The search bar is also more prominently placed.
Goodreads, which Amazon acquired back in 2013, has been a fixture in the Kindle app for quite some time, allowing you to mark books as read on your Goodreads account and to share what you’re current reading with your followers. But starting today, Goodreads is more deeply woven throughout the mobile app’s entire look and feel."Amazon’s new Kindle app adds a light theme and deeper Goodreads integration - The Verge