Comment 8 for bug 1969984

Unfortunately it's not that simple. All legacy full-screen Plymouth themes are affected by this bug. In order to work correctly, the Plymouth theme is now relying on an existing parent theme as plymouth-theme-ubuntu and plymouth-theme-ubuntustudio: plymouth-theme-spinner. This parent theme relies on the ACPI to communicate the screen dimensions as opposed to assuming screen dimensions from a random sampling of one display, which is how the old method worked AIUI. This is error-prone when all displays are active as opposed to simply display 0 in the case of legacy themes.

TL;DR: a small change to the existing theme would never work.