Customizing themes in Drupal 6
This is just a quick guide assuming you know what you're doing, but haven't done it for a while and need some quick notes.
First, copy your base theme into its own folder in the themes folder.
Rename the info file to match your new theme name, and edit the file appropriately.
Edit the page, node, and css files as desired.
Choose your theme from the admin menu, and there you go!