This is a biginner tutorial writtin by a beginner(not a good speller at that!)
This is how i started to gain knowlege on skinning gimp (the windows version).
Step 1) Viste themes.freshmeat.net
and download the Mechanics theme Mechanics
step 2)copy contents of archive to C:\Program Files\GIMP\share\gimp\1.2\.gimp-1.2.User or C:\GIMP\share\gimp\1.2\.gimp-1.2.User (depending on where gimp is instaled to) and allow the archive to overwrite files.
step 3) start gimp and view your newly skined gimp.