Combining Stylesheets

[fairestcape]

so why don't the people that make the mods just add on to the main stylesheet of zen cart?

... because the stylesheets are one of the most "customizable" aspects of the program... they don't know how or where users have modified their stylesheet, and in order to provide a (relatively) easy way of making comparisons between a mod stylesheet and the user's stylesheet, keeping things separate makes sense.

As indicated, there is nothing wrong with "blending" stylesheets, as long as there is a good understanding of how stylesheets behave (chronology, structure, order, etc).