styling my marketplace

I've got my marketplace built

Now I need to style it. As I go through it, I find that I can't isolate the elements enough without also styling more core elements of the theme (both the child and parent theme). Even though I've offset my marketplace and sidebar in their own div classes, I can't get control over them.

I know the answer is an external css stylesheet which is then called using the template for the marketplace. How do I call an external sheet? Is it done by adding arguments to get_header()? and how is it done to ensure the primary stylesheet is loaded and then the specialized stylesheet is loaded to over-ride it?