Re: style vs screen CSS
http://codex.wordpress.org is probably the best place to start when trying to figure out what functions perform what tasks for what purposes. Also, think about using an IDE like NetBeans so you can reference functions and trace their usage to get an idea of why you would or wouldn’t want to use a specific function, action, or filter.
WordPress has a steep learning curve, and it uses an action oriented code structure rather than an object oriented one. There are objects out there, but you rarely interact with them directly, and in the WordPress world interacting with those objects is typically left to plugin developers to tinker with, and best left out of the template files.
Since you’re just learning, the best advice I can give you is not to edit any “core” file, or rather any file that comes included in WordPress or any plugin that you might use. If something isn’t doing exactly what you want it to, chances are you can adjust it somehow. If you can’t adjust it, make your own plugin and copy/paste the code that isn’t good enough and make your own functions out of them. If someone would have told me that 3 years ago I would have been a much happier person then.