This is fairly different from say Blueprint/Compass where almost anything can be tweaked just with SASS varaibles.
I'd go a step farther and say that even the little bits you can change are prone to abuse and/or simple hard to get right for a number of non-designers. The generator system above actually just fills in some blanks, which isn't exactly the difficult part for programmers. It's what you fill in those values… My idea for some part of "custom" generator would be selecting a palette from a reasonable large number and then probably some basic font types and weights, similar to type-a-file[1]. Then add some customized icon set[2][3], and you've got something that's at least a bit more unique.