GTK Themes, and theme libraries : how do they fit (get plugged) into GTK ?

[Philippe Bertin <philippe bertin telenet be>]

Hello, all,

Some time ago I asked a question on a library that hadn't been found 
upon installing a new GTK library (libqtengine.so, that was). I slowly 
start realising now that this was a file belonging to what's called a 
theme library, and that it is actually not an integral part of GTK 
itself (so typically part of a distribution, I guess)

Then I asked myself how this theme mechanism is working in GTK. I found 
out that there's a whole lot of an object's properties that are 
customisable in gtkrc files. But the changeable properties in these 
files can (unless I'm wrong, or wrongly understand the code) at most 
explain some differences in colors, but not at all any difference in 
widget drawing method.

So how(/where in the code) are these theme libraries coming in the game 
? How can e.g. a GtkToggleButton possibly be drawn another way, e.g. 
with more rounding, or with another kind of shading (as I said, I'm 
explicitly *not* speaking of a different color scheme) ? How(/where in 
the code) do these theme libraries get "plugged-in" into GTK ?

Could someone please give some beginning of conceptual explanation, and 
then give further (clues to) links to good documentation on how this is 
all working together (a hint in the GTK code can also do) ?

Thanks, and kind regards,

PhB.