There are 3 choices for the alternative default.plymouth (providing /usr/share/plymouth/themes/default.plymouth).

  Selection    Path                                                                     Priority   Status
------------------------------------------------------------
* 0            /usr/share/plymouth/themes/ubuntu-gnome-logo/ubuntu-gnome-logo.plymouth   150       auto mode
  1            /usr/share/plymouth/themes/ubuntu-gnome-logo/ubuntu-gnome-logo.plymouth   150       manual mode
  2            /usr/share/plymouth/themes/ubuntu-logo/ubuntu-logo-scale-2.plymouth       99        manual mode
  3            /usr/share/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth               100       manual mode