Best approach to style pages by category

Create Category templates where you could add CSS ID’s or classes etc to be able to style respective categories differently

Right, that’s how I do it for category pages. But how can I style post pages, “single.php” differently depending on what category the post is in?

I see you’re using WP 2.8. Unknown to some, there is a new body_class tag which will allow themers to add more style to certain posts. Check out
https://wpengineer.com/wordpress-28-body_class-automatic_feed_links/
https://www.nathanrice.net/blog/wordpress-2-8-and-the-body_class-function/

There’s a plugin but haven’t tested it for 2.8
https://www.ads-software.com/extend/plugins/post-template/

In your single.php file using conditional tags:
https://codex.www.ads-software.com/Conditional_Tags
https://codex.www.ads-software.com/Template_Tags/in_category

Great, I’ll look that stuff over. body_class sounds like a winner.