The phrasing of your question is a little unclear: I'm assuming you mean the first set of CSS is yours and the second is the existing CSS?
In that case, the problem is being caused by the !important. !important causes that CSS rule to override any CSS rule that comes before or after it.
The only way to override CSS with !important is to put your CSS after it, also marked with !important.
http://coding.smashingmagazine.com/2010/11/02/the-important-css-declaration-how-and-when-to-use-it/