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If you use a debugger such as firebug for firefox or developer tools for IE, or debugger in safari or google chrome, just right click and select “inspect element” It should point you to the css file and also the class or Id you would like to change!
And I would highly recommend using a custom template so you can upgrade Magento as new versions come out with little difficulty. Also, after playing with my custom theme for a while, I actually found it easier to do things like add custom CSS and JS files, as well as mess with the phtml and xml. It has been a lot less buggy with the custom theme