To go back to previous grub background
If you want to go back to the Ubuntu's login screen and grub background, use this command
sudo update-alternatives --config text.plymouth
This will show you options to select something like below -
There are 5 choices for the alternative text.plymouth (providing /usr/share/plymouth/themes/text.plymouth).
Selection Path Priority Status
- 0 /usr/share/plymouth/themes/xubuntu-text/xubuntu-text.plymouth 150 auto mode
1 /usr/share/plymouth/themes/kubuntu-text/kubuntu-text.plymouth 60 manual mode
2 /usr/share/plymouth/themes/text/text.plymouth 40 manual mode
3 /usr/share/plymouth/themes/ubuntu-mate-text/ubuntu-mate-text.plymouth 100 manual mode
4 /usr/share/plymouth/themes/ubuntu-text/ubuntu-text.plymouth 50 manual mode
5 /usr/share/plymouth/themes/xubuntu-text/xubuntu-text.plymouth 150 manual mode
Select the one with Ubuntu by typing the number and press Enter.
To remove most of the KDE and plasma components
Use the solution I gave in this question https://askubuntu.com/a/814627/61218