sudo apt-get install --reinstall plymouth-theme-ubuntu-logo

sudo update-alternatives --config default.plymouth

There are 4 choices for the alternative default.plymouth (providing /lib/plymouth/themes/default.plymouth).

  Selection    Path                                                           Priority   Status
------------------------------------------------------------
  0            /lib/plymouth/themes/numix/numix.plymouth                       100       auto mode
  1            /lib/plymouth/themes/numix/numix.plymouth                       100       manual mode
* 2            /lib/plymouth/themes/solar/solar.plymouth                       10        manual mode
  3            /lib/plymouth/themes/ubuntu-logo/ubuntu-logo-scale-2.plymouth   99        manual mode
  4            /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth           100       manual mode

Press enter to keep the current choice[*], or type selection number:

sudo update-initramfs -u