The BGRT theme is probably a close as to "FlickerFree" we can
get without https://github.com/NixOS/nixpkgs/pull/74842.
It's more agnostic than the Breeze theme.
We also install all of themes provided by the packages, as it's possible
that one theme needs the ImageDir of another, and they're small files
anyways.
Lastly, how plymouth handles logo and header files is
a total mess, so hopefully when they have an actual release
we won't need to do all this symlinking.
6bd4f9a3c5