All themes have pretty much the same elements in them, that's why people use someone elses theme then modify it to there own use, it's the quickest way possible to learn and to make one.
Once you got all the elements down then make it your own, but all themes need certain elements for them to work, so it really doesn't matter which one you use, as long as it has the elements needed then make the changes you want to it.
If your using Mate and making gtk2 ones there is a theme creator (doesn't make it for you and frankly I found it aa waste a time) but alot of folks liked it, it can be found here:http://gnome-look.org/groups/?id=377
not sure what gtk this works on anymore though, they used to have it for gtk2.
If you want to get down and dirty heres the whole shabang for gtk2 themeing: https://live.gnome.org/GnomeArt/Tutorials/GtkThemes
I lived here at this place to learn themeinjg in the beginning. Its for gtk2
if you want gtk3 well thats a whole different ball game because gnome flipped it to css instead but a lot of it is similar in the gtk still, just have to figure that out.
But here is some gtk3 tutorials and references for you: https://developer.gnome.org/gtk3/stable/
The other thing is what shell is it using? it usually has three parts now, a shell (gnome 3, unity (cough), cinnamon (which I perfer) or just gnome 2 stuff) then the gtk (the guts inside your window your looking at (all the stuff that happens usually inside a window frame) then the window frame itself, there it depends on what your using, if it's mate then it should be metacity (it uses xml to frame) and you just add images and code it in, all metacities have pretty much the same elements but you can add and take away stuff and make some nice frames with it, for xfce there is xfwm4 frames (easier then the metacity but along the same lines, there is quite a few more also, plus then there is adding of gtk engines like murrine, pitbull (I think thats what it's called) and other add ones that folks made through the years.
So to say show me how, well like coding anything there is quite a few ways to go at it and like most codes things get added on etc etc, it's really not a 1,2,3 step way of doing, but you got enough to get you going through the links.
But a word of advice, take one you like tear it apart to see how it was made, most have the same elements and all have a certain amount of base elements needed or it won't work, and make your own. It's the quickest and easiest way to learn.
this place https://live.gnome.org/GnomeArt/Tutorials/GtkThemes
will tell you what everything does and also what extra to add to some softwares like gimp, banshee etc
Hope it helps you out on your journey.