We like to use stylesheet_zcustom.css as it will most often load last. Stylesheets load alphabetically with stylesheet.css always loading first. So, if we used the mycolours(sic) stylesheet and later added a normal_colors stylesheet, the normal_colors would load last and take precedent. If we added a main_colors stylesheet, the mycolours(sic) would still prevail.

One extra point is that responsive stylesheets often load after their alphabetical "order". So, even using stylesheet_zzz.css, you might find the occasion where a call in this file is "taken over" by one of the responsive stylesheets. This is where one might use the !important property to override subsequent rules.