my lightdm became curious look like in the picture. how to restore the original look like Ubuntu?
You need to install unity-greeter and configure lightdm to use it. You are running the gtk greeter now for some reason. When lightdm is mis-configured it enters "low graphics" mode. You can confirm the reason by looking at /var/log/lightdm.log
Make sure unity-greeter is installed. You can easily do this by just reinstalling it with apt sudo apt-get install unity-greeter.
Edit lightdm.conf, it should look like this (below), the key line being "greeter-session=unity-greeter"
After you do that, start lightdm from a virtual terminal. sudo initctl start lightdm. I'm not sure if lightdm is "running" when in low-graphics mode, if so, stop it first with sudo initctl stop lightdm.
Note: You do NOT need to uninstall the lightdm-gtk-greeter, just leave it for a backup or once everything is working, then you can remove it if you want.