i doubt you'd have to edit the .gui files - i didn't - they're all just recolors/redone images at pretty much the original sizes. The Shell.zip i posted has all of them in it.
Looking at Shell.vol in 1.3 it looks like they're in .bmp but knowing tribes its probably PBMP
so i'm assuming you'd have to convert all the .png files in my Shell.zip to pbmp and then put them in Shell.vol for 1.3 (but i dunno maybe with hudbot u can make them targa instead?)
overall probably more of a pain in the ass than it's worth
p.s. i updated the
Shell.zip in March with a few fixed buttons
edit: no longer includes keyboard cat