add rules into an xsl file

To add a rule into an xsl file so that depending on the rule it would apply a different style An example is the xsl chooses a background for the page depending in the information supplied in the data by using <DIV id="DataBGShoww" ></DIV> and DataBGShoww is defined in the css stylesheet. I need to add a rule where when the information changes in the data it chooses a different background. I have access to the current xsl, the css files and the data so just need someone to add the rule to make it choose a different stylesheet. This is a small job as it really is knowing how to add that rule using <xsl:if test I assume I am ideally looking for this to be done immediately as in now