To test your custom theme's CSS we have a themer kit which can be downloaded (Note: remove the .txt extension and then decompress). The kit contains a dummy mini-site of files with typical markup and with links to the live base theme, as well as links to a blank css file called my-theme.css. This is where you should work. Add background images to the supplied images directory.
The files may be dropped onto Firefox or another standards-compliant browser to view as you work. (Note: We've found that Dreamweaver does not accurately render the base CSS.)