You can use: plymouth-set-default-theme solar --rebuild-initrd

Like this:
[autopsy@localhost libexec]$ su -c 'plymouth-set-default-theme solar --rebuild-initrd'
Password:
[autopsy@localhost libexec]$

[autopsy@localhost libexec]$ ls -l /boot/initramfs-2.6.32.11-99.fc12.i686.img
-rw-r--r-- 1 root root 11709912 2010-04-25 11:45 /boot/initramfs-2.6.32.11-99.fc12.i686.img
[autopsy@localhost libexec]$

11:45 AM, when I issued the command the initrd was rebuilt with my new Solar theme change.
(This assumes your theme is packaged properly, this will work.)

On Sun, Apr 25, 2010 at 10:49 AM, Sebastian Dziallas <sebastian@when.com> wrote:

Ray Strode suggested that this might be caused by the fact that the
plymouth theme gets installed past the kernel in the build process,
and that if this was the case, it'd be required to rebuild the initrd
in %post. Turns out that sugar-logos actually gets installed past the
kernel. But even after calling dracut in the kickstart's %post part,
nothing changes.