YouTube tests new versions of the homepage and video pages, but also a new video player. The new interface replaces the gray shades with pure black, updates the icons and tweaks the progress bar color.
For some reason, the old interface was more subtle, while the new one seems to be more in-your-face. Now that YouTube tests a white background for video pages, the new player stands out more.
Here's the experimental player:
... and the player from the standard interface:
To try the new player, change your YouTube cookies using the instructions from this post. There are some other changes: new options in the "upload" drop-down, "now playing" is replaced by "what to watch", YouTube now shows the number of subscribers and there's a new way to display the number of likes and dislikes.