Customizing a CSS stylesheet

I am working on a product template page with a custom stylesheet. I am having problems uploading my custom css to a replicated website due to the fact that I can't edit the higher level stylesheets. This is a replicated website where I am able to add custom html and a stylesheet, but I am unable to edit higher level style sheets. I am having trouble with the spacing and the overriding stylesheets and need help creating a lower level stylesheet where the classes won't be over-ridden.