After installing some other desktop environment like Kubuntu etc. Your GRUB loader automatically gets changed. For example after you install Kubuntu via apt-get command, it automatically changes default Ubuntu Boot Loader to blue plasma one. This tutorial explains you how you can gte back to old boot theme.

So first thing you ned to do is click Dash Home icon present on the top of Ubuntu Unity Launcher, type terminal in search box and open it.

Now run the following commands as described:

sudo update-alternatives --config default.plymouth

Choose the option you want.

Followup this with following command:

sudo update-initramfs -u

Now run

sudo update-grub

That’s all. Reboot your machine. It also sets default Ubuntu login screen back.