Like any other window of any other application, if it’s in the way, either close it, if you don’t need it, or Minimize it if you want to keep it open but want to hide it. Like Raino said, it’s no different than any other window. It’s just Cubase without a project to display, so there is only the menu bar.
Oh, that’s not controlled by Cubase that’s in your Windows colors settings. The bar turning white is Win10 indicating that a Window (in this case Cubase) doesn’t currently have focus. Other Windows do the same thing but it is more noticeable when it is just a bar without a background. Check out the attached of a Control Panel Window doing the same.