I've done a few art packs myself, simple way is to create a new folder in the 'art' folder and name it what ever you like, then go into one of the default art pack folders and do Ctrl-A, Ctrl-C. Go into the new folder delete what's there, then simply Ctrl-C will copy the contents into the new folder. Now open the floor, wall, sky, etc, image files in simple M$-Paint and edit as needed.
Thank you! I've now done exactly the same thing and created a whole slew of art packs.