Hi,
As has been said above, Ubuntu is Debian with newer versions of some stuff, a different install program and a pretty desktop.
I’ve had what you experienced happen on one of my machines when I installed a different desktop. I installed something that hadn’t been Ubuntu-customised and that caused the boot screen to revert to the original one it had before Ubuntu applied their look. Is it the boot screen that has changed (the one that shows up when you turn the machine on) or the login screen (the one you type your password into)?
There’s almost certainly nothing to worry about. Nobody has compromised your box.