Zencart loads all stylesheets whose names begin with 'style', have the .css extension and are in the current template's css folder. Stylesheets are read top to bottom, and (I believe) are loaded alphabetically by file name. If you have a rule in stylesheet_main.css and the same rule in stylesheet_tm.css, the latter will prevail, as it is read after the first and will 'overwrite' the rule from stylesheet_main.css.

p.s. If you can 'optimize' a Template Monster template, you're a better man than I.