When I restarted my Elementary OS Laptop today I was shocked, because I was on an Ubuntu-styled glitching Login Screen. I guess I accidentally installed it, by trying to get ubuntu-tweak-tools on my Laptop. I need to get my old Elementary OS Login-Screen back. (All the other things are still elementary. Only the login screen changed it style)
First of all,
unity-tweak-tool is not suited for tweaking elementary OS. Some things might work, but it's completly unsupported as it is written for tweaking the
Unity desktop environment - elementary is using
Pantheon, a desktop written from scratch.
unity-tweak-tool pulled in a package called
unity (a metapackage), which in turn pulled in core packages for the Unity DE. You don't need most of these packages, but it won't hurt you either, so there is not a real problem - save for a package called
unity-greeter. This is a greeter for LightDM (the software behind your login screen) and it overwrites elementary's default greeter.
To remove the greeter, just run this command from terminal:
sudo apt-get remove unity-greeter