Most APEX developers will eventually get asked to brand their application or "make it look nice". If you are like myself, and are not a graphic artist, this can be quite a challenge since it is an art to make a web pages look really good.
I've had to do read up on it over the past few months. I finally read an article on CSS specificity (try say that 5 times in a row :-). CSS Specificity is like putting a score or weight on each CSS definition so you know which ones will "override" other definitions.
Here's the article: http://www.smashingmagazine.com/2007/07/27/css-specificity-things-you-should-know/
There's also a link to the CSS specificity calculator: http://www.rebelinblue.com/specificity.php