Added Corner Banner

Im trying to add a corner banner but am having a bit of trouble. Do I add the css code to the style.css and it will automatically show up on every page?

Here is the code:

#topright {
position: absolute;
right: 0;
top: 0;
display: block;
height: 125px;
width: 125px;
background: url(http://www.mysite/images/banner.gif) no-repeat;
text-indent: -999em;
text-decoration: none;}