If you are using a CSS file for your project, I think you can
change all tables with that. However, you would have to do it
manually because the closest thing RoboHelp's style sheet dialog
lets you do is state a style for table text. CSS fans will tell you
that the point of CSS is to avoid using tables for layout, but you
can still use CSS to dictate how your table looks. A few extra
lines in your CSS file, and you should be able to change all tables
without making any changes in each topic.
--Ben