Learnt something today, when playing around with a new theme - if you want to clear the contents of an active style element in an alternate stylesheet, simply use null as your value. For example, with the 'flower' stylsheet, I needed to change the #title ID... the base stylesheet specifies a right-hand positioning of the title element... and I wanted to move it to a left-hand positioning. In order to do this, I had to enter the following:

#title {
  right: null;
  left: 10px; }