如何在Centos 6.3中更改登录屏幕背景

我想在Centos 6.3中更改默认的登录屏幕背景/主题。INSTALL我下载的主题中的文件夹显示：

Use 'gdmsetup' to install.

但这gdmsetup不是公认的命令。我安装了最新的EPEL存储库，并尝试了一个yum install gdmsetup无效的存储库。

我做到了 yum list *gdm*，我得到了：

[root@localhost BlueSwirl]# yum list *gdm*
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
 * base: mirrors.syringanetworks.net
 * epel: mirror.itc.virginia.edu
 * extras: centos.aol.com
 * updates: mirror.symnds.com
Installed Packages
gdm.x86_64                         1:2.30.4-33.el6_3.2 @updates                 
gdm-libs.x86_64                    1:2.30.4-33.el6_3.2 @updates                 
gdm-plugin-fingerprint.x86_64      1:2.30.4-33.el6_3.2 @updates                 
gdm-user-switch-applet.x86_64      1:2.30.4-33.el6_3.2 @updates                 
plymouth-gdm-hooks.x86_64          0.8.3-24.el6.centos @anaconda-CentOS-201207061011.x86_64/6.3
pulseaudio-gdm-hooks.x86_64        0.9.21-14.el6_3     @updates                 
Available Packages
gdm-libs.i686                      1:2.30.4-33.el6_3.2 updates                  
gdm-plugin-smartcard.x86_64        1:2.30.4-33.el6_3.2 updates                  
lcgdm-dav.x86_64                   0.11.0-1.el6        epel                     
lcgdm-dav-devel.i686               0.11.0-1.el6        epel                     
lcgdm-dav-devel.x86_64             0.11.0-1.el6        epel                     
lcgdm-dav-libs.i686                0.11.0-1.el6        epel                     
lcgdm-dav-libs.x86_64              0.11.0-1.el6        epel                     
lcgdm-dav-server.x86_64            0.11.0-1.el6        epel                     
lcgdm-devel.i686                   1.8.6-1.el6         epel                     
lcgdm-devel.x86_64                 1.8.6-1.el6         epel                     
lcgdm-libs.i686                    1.8.6-1.el6         epel                     
lcgdm-libs.x86_64                  1.8.6-1.el6         epel                     
nagios-lcgdm.x86_64                0.9.4-1.el6         epel                     
nagios-plugins-lcgdm.x86_64        0.9.4-1.el6         epel                     
nagios-plugins-lcgdm-common.x86_64 0.9.4-1.el6         epel                     
root-gdml.x86_64                   5.28.00h-2.el6      epel                     
[root@localhost BlueSwirl]# 

我不确定我缺少什么或需要做什么。谁能告诉我如何自定义我的登录屏幕或使用某种主题安装程序对其进行自定义？