Actually that's exactly what the Zen Cart team did do for the first few releases of 1.3.x, specifically to ease the upgrade path for existing store users.
But this made life much more difficult for new users (and confusing for existing users too), so given the extent of the practical problems it was causing, they moved forward with 1.3.5 and re-integrated the styles into a single sheet for the classic template.
It might help you to download the release pack for 1.3.0.2 and look at the split between stylesheet original and stylesheet_new, but I doubt it. I must have upgraded a couple of stores from 1.2.x to 1.3.x and I've yet to see one where it wasn't easiest to simply rebuild the template from scratch using the much more powerful and flexible XHTML / CSS architure to which Zen Cart has evolved.




