By using a template that is well designed (or by building your own), you will have gone a long way towards having a more accessible website before you even write a line of code or add any content.
Note
There are several sites that offer high quality commercial templates for Joomla! that are graphically sophisticated and have been designed to be accessible such as Yootheme (http://www.yootheme.com) and Rocket Theme (http://www.rockettheme.com/>).
So what are the advantages of using Joomla! templates?
There is a complete separation of content from presentational elements. Using CSS for layout means that you are using the right tool for the right job. Leave the tables for your tabular data and nothing else.
One of the greatest benefits of CSS is that by changing a rule in the CSS declarations, you can change the entire look and feel of all of the elements of your site that use that rule.
This makes site design and maintenance so much easier than having to individually...