Zen's table styling fix breaks update module's styling

Core has god-awful default styling. Zen has had a fix for that; it resets the default table styling, but then re-applies them to all tables in the admin form. This breaks update.module's styling for its "Available updates" page; you're missing all the nice green and red background coloring because Zen has applied a more-specific rule that has grey background.

I think we can fix this by changing Zen to only apply the core table styling to FORMs that have tables.