With macOS Tahoe, Apple is introducing a new Liquid Glass look for the user interface. This new look involves transparent elements where some sections on screen will be able to show elements layered below them. It gives macOS a fresh, new look. (Or makes it look a little like Windows Vista, if you’ve been using computers long enough to understand that reference.)
If you don’t like the new transparent look or if it affects how well you can see items, there is a way to reduce it, and in some instances, turn it off. Here’s how to reduce transparency in macOS Tahoe.
Read more at Macworld.com