If you know HTML and CSS, you're fully equipped to edit the error page themes you can add to your site with Website Access Manager. To do so, first install the themes in Website Access Manager. For more information about how to do this, read this article: How Do I Set up Custom Error Pages?
Next, use use CoffeeCup HTML Editor (or any other HTML editing program) to open and edit the HTML pages from these locations:
C:\Documents and Settings\[username]\My Documents\Coffe
If you would like your changes to be reflected in the preview area of Website Access Manager, you will need to take a screenshot of your changes and save it using the same filenames as found in the folder containing the theme.
Pro Tip: It's a great idea to link to your sitemap from any error pages, just to make sure your users can find where they're trying to go. Don't have a sitemap? Good gravy, why not?! Check out CoffeeCup Sitemapper for the easiest way to create beautiful, comprehensive sitemaps for your website.