Your CSS code should work if you add it to the end of the "additional.css" template for that particular parent style. This is where you would normally add CSS snippets to override vbulletin CSS.
Sometimes you can get away with just altering the vB-STYLEVARS for minor changes. For small amounts of CSS code there is virtually no performance difference, but there is advantages and disadvantages for both. Nothing that I would worry about at this point.
@Lynne is one of the CSS gurus around here, I tend to follow her guide lines.