Open the terminal and run the following commands
sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
Then logout, and you’ll see an Appearance window pop up. Change it to how you prefer it, then close it and login as usual.
When you have logged in after finishing the customizing, run this command to prevent the Appearance window from opening at the GDM screen every time.
sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop
If you want to install plymouth themes run the following command from your terminal
sudo aptitude install plymouth-theme-*
Now you can run the following commands to change login and plymouth screens
sudo update-alternatives --config default.plymouth
gksu -u gdm dbus-launch gnome-appearance-properties