If you want to change your textures and want your inventory slots to match your new textures, but don't want to make new slots for every texture ...
These inventory slots have a transparent background and semi-transparent edges, so you will see the background texture through them.
I have a blank inventory slot that matches, but it's in window_pieces01.tga, so you would get a whole lot of other stuff you probably don't want. Only install that file if you *really* need the transparent inventory slot.