Change font size in CSS Stylesheet

Hi
I have been struggling for hours just to change a font size on my pages. What a mission to do this on a customised theme! It does not work by changing the font size specification in the body tag as shown below:

body
{
  margin: 0 auto;
  padding: 0;
  <strong>font-size: 80%; /* Resets 1em to 10px */</strong>
  font-family: 'Lucida Grande', Verdana, Arial, Sans-Serif;
  background-color: #123C66;
  color: #000000;
}

I got it to work changing another tag, maybe this will help someone else with the same problem :

.BlockContent-body
{
  position: relative;
  overflow: hidden;
  z-index: 0;
  margin: 7px;
  text-align: left;
  color: #085159;
  font-family: 'Arial';
 <strong> font-size: 12px;</strong>
  font-weight: normal;
  font-style: normal;
  text-decoration: none;
}