you would not find a theme folder because the Original Cairo clock does not have one.. The color and the transparency are brought about essentially through alpha channels in Cairo using such code as
The single most important difference between Cairo Clock and SVG Clock is that I have enabled SVG rendering support in the SVGClock. This basically enables different themes/clock face rendering for the clock.

You find the SVGClock 'chunky' or slow compared to the Cairoclock, because in the SVG clock, different SVG files/clock parts ( clock face, minute & seconds hand etc) are getting rendered real time compared to simple Cairo lines and arc generated in case of Cairo Clock

You can get an 'acceptable' level of performance in SVGClock by disabling the second hand ticking ( reduces CPU cycles), as mentioned in Post #30 of this thread.