The pages, images and news articles are all OK, but the {stylesheet} tag returns nothing (empty string) so the site looks messy.
No error messages in PHP Log or PostgreSQL Log.
In "Content"/"Pages", the pages still have templates attached.
In "Layout"/"Templates", the templates still look OK: content area and stylesheet are filled in.
The template content area reads:
...
{stylesheet}
On the old site, {stylesheet} expands to
But on the new site, {stylesheet} expands to nothing

Also, if I edit the template and change {stylesheet} to by hand, the page still doesn't render because http://localhost/stylesheet.php?templateid=21 returns 0 bytes of data.

If I set $config['debug'] = true in config.php, then I can see this SQL query:
SELECT template_id, template_name, template_content, stylesheet, encoding, active, default_template, modified_date FROM cms_templates WHERE template_id = '21'
... which works OK if I execute it at the SQL command console.
I've tried $config['old_stylesheet'] both true and false in config.php.
Has something very important changed with templates & stylesheets in the last few releases?

Thanks for any pointers!
- Martin.