WIN XP
C:\Documents and Settings\your username\Application Data\LimeWire\themes

Application Data's hidden by default, so you'll need to show hidden files & folders in order to see it (control panel > folder options > view > check the radio button to show hidden files & folders > apply > ok).


VISTA
C:\Users\your username\AppData\Roaming\LimeWire\themes

The App Data folder's hidden by default, so check out the thread below about how to show hidden files & folders & how to find the preferences folder in Vista (the preferences folder is named LimeWire & the themes folder should be within that).
LimeWire Pref Folder in Vista