You can preview YouTube's new desktop UI today as YouTube prepares to introduce users to a brand new experience with the user interface. There are a few things to make note of with these changes and that's first and foremost that YouTube is still very much working on this new site design, which means that it could still be a few months before it's completed and they roll it out. The good news is that today YouTube is allowing anyone who wants to give the new design a try the opportunity to do so by opting into the preview of the new design, meaning you don't just have to stare at pictures of the changes and wish you could apply it. Since this is just a preview though and the design is still a work in progress, things are subject to change and YouTube could tweak some more things visually before everything is final.
If you opt into the preview you can switch back to the old YouTube UI pretty easily from the account settings menu if you prefer it, and you'll be able to use the old UI up until YouTube rolls out the new site, though they haven't mentioned an exact date for the roll out yet. As for the visual changes, the desktop UI is now draped in white across the page with less red, and there are bits and pieces of familiar Material Design elements that you might pick up right away while others may slip by unnoticed. For example the new UI now incorporates the hamburger menu to the left, which gets rid of the side bar where you find things like your subscribed channels, your history and other things. You can hide this on the main home page, but you can also bring it up when inside of a video as it's hidden by default on video pages.
YouTube has also now applied a dark theme option which you'll find a toggle for from the account settings. Accessing account settings is done by clicking on your account icon so there shouldn't be much of a change here. With the dark theme on YouTube takes on more of a visual appearance of YouTube Gaming which already uses a dark colored theme, and certain elements while the dark theme is applied are more noticeable, like the Home, Trending, or subscription buttons which will be highlighted in red when one of them is selected, though this particular details shows up with the light theme also. YouTube hasn't just made changes to the home screen as there are visual updates to video pages too. Subscribe buttons appear much more prominent than before and are not only bigger but they appear to the right of the video instead of being on the left and right under the video name. They're still a bright red though while they switch to a light grey after you subscribe to a channel.
The comments section has been updated a little bit as well as the "add a comment" box is no longer in the outline of a chat bubble, and instead is just a line that sits above the comments. The whole page is also now using less of a defined separation between sections. On the older style of the YouTube UI each section is represented by its own card similar to the way things look within the Google app, but no longer. Instead everything is presented as a continuous flowing page, though elements are separated by a slim line. YouTube notes a few different reasons for making these changes. For one, the UI is now streamlined and unified across all platforms and matches up with the YouTube mobile app. They also state that simplicity is a big key factor as they wanted to offer less distraction from the most important thing which is to watch the videos, hence the removal of some of the visual elements that used to be there, like the cards that would separate sections. Everything now has a more cohesive and less withdrawn appearance to it. While there aren't many new features included with the new look, save for the dark theme option YouTube boasts more "new, powerful features coming soon," which could be introduced before the final release of the new UI or they may launch with it.