Open the page in KompoZer.
Click on the artist's palette in the tool bar to open the css editor.
At the upper left click on the dropdown arrow next to the artist's palette and select 'linked stylesheet'.
On the dialog box that opens click the select stylesheet button and navigate to the styles.css file.
If the 'internal stylesheet' is still in the left pane of the css editor select and delete it.
You should be good to go.
There are a couple of helpful resources that you should know about:
Charles Cooke's KompoZer Users Guide: http://www.charlescooke.me.uk/web/kz-ug-home.htm
Gregtutor's tutorial content at: http://www.gregtutor.plus.com/webdesign/index.html