I preferred it without the border, but the way it is now isn't too much different. I'm good with it.
I never have my browser window fullscreen, but I notice if I do maximize it, the border becomes much wider while the forum part stays the same width. Conversely, if I resize my browser window to about half the screen width, the border disappears and the forum part gets cut off so you can't read it all. I don't remember if it was like that before, but I just checked another forum (uses vBulletin, not Simple Machines), and it doesn't work like that. The forum has a narrow border, which stays constant even when maximized, and no matter how narrow you make the browser window (up to about 1/4 screen width anyway), the text just wraps so you can read the whole thing.