You are needing to get back to:
C:\Windows\Resources\Themes\Luna\luna.msstyles
Does the color change if you select Ctrl/Alt/Delete/Task Manager? If so
use: File/New Task (Run)
Restore Luna Theme (Line 187)
http://www.kellys-korner-xp.com/xp_tweaks.htm
Restore Themes Functionality (Line 187)
http://www.kellys-korner-xp.com/xp_tweaks.htm
Troubleshooting Themes
http://www.kellys-korner-xp.com/xp_t.htm#themes
http://www.kellys-korner-xp.com/top10faqs.htm
If this isn't a possibility, load your hive via the System Admin to correct:
1) Click Start, Run and enter REGEDIT
2) In Regedit, highlight the HKEY_USERS key and go to File, Load Hive.
3) Use the File Open dialog to go to the Documents and Settings\<username>
folder, where <username> is the account you wish to modify.
4) Highlight the NTUSER.DAT file in this folder (usually a hidden file) and
select Open.
5) You'll be prompted to enter a "Key name". You can use whatever you wish,
but I use the User's logon name.
6) You can now expand the Hive you just loaded and make any needed changes.
7) When finished, highlight this Hive again and go to File, Unload Hive.
NOTE: You MUST unload the Hive prior to logging on to the users account.
Otherwise XP may have trouble loading the user's profile.
Good luck and keep us posted.