Are there any good software for designing, or helping in designing, GTK+ themes for use in Gnome and Xfce desktops?
I don't know of any such software. I think it has to be done "by hand". Try this link.
I know this question is an old one, but I happened upon an answer for this, so I thought I'd share. oomox seems to be the best way to do this, short of doing it by hand (which I'm sure is a learning experience worth trying, but this tool makes it easy)
To quote the description in their README:
Graphical application for generating different color variations of Oomox (Numix-based), Materia (ex-Flat-Plat) and Arc themes (GTK2, GTK3, Cinnamon, GNOME, Openbox, Xfwm), Archdroid, Gnome-Color, Numix, Papirus and Suru++ icon themes. Have a hack for HiDPI in gtk2.
It also seems to be compatible with most Linux distros and most DEs.
Not really an answer to your question, but if you have the patience you could wait for the new theme solution based on CSS to complete. More information here.
One method which I found to be not so bad is to take a theme and modify the gtkrc file.
You'll have to experiment and do a lot of guessing and trial-and-error, but it worked for me. All I really wanted was to make that theme less gray-ish and a bit more silver-ish and blue-ish.
I am working on a program that will help you see what your changes to gtk or metacity themes look like instantly. Check it out here.
I didn't find one either.
There's GNOME Color Chooser, which may help.