Changing CSS in Zen Theme in Drupal 7
You should just focus on several main Zen css files:


blocks.css : should add the css of blocks that you create from backend or from your modules.  
forms.css: should add the css for forms: for example: login form, register form, contact form (from contact module)  
pages.css: should add the css for the pages: static page that you create from Drupal backend or views page..  
nodes.css: should add the css for just the common structure of node and specific nodes also  
tabs.css: should add for the tab style (You can easy see the tab style at user login page with login/register/forgot password).


For Zen theme. I suggest you should understand SASS structure and change the scss files instead of css files and the name of scss files are the same as the list above (like blocks.scss, pages.scss, nodes.scss, tabs.scss). You should check Getting started with Drupal7, Zen, Sass and Compass for doing your current stylesheet.