In the Beginners CSS section  you learned some things about why CSS has the word cascading in it.  Basically later styles over-rule earlier ones.  So external styles are easily over-ridden by in-line styles.

However there is another rule to confuse that cascading.  More specific selectors will take priority over less specific ones.  Imagine this:

A p selector is a more specific selector than the in-line style in body element.  Therefore all paragraphs will be white.  The more general body selector will still apply to heading elements and so on.