Design Manuals

Do you ask this as a user or a designer? The changes to the public side of WordPress were really small, so older themes should often work. Design is merely the same (apart from the administration panel, of course).

The only adjustments made to the theme system for WP v2.0 were the added possibility of a special theme-specific plugin-like settings page and a thumbnail for each theme to display in the Presentation menu. More info: https://asymptomatic.net/2005/12/19/2171/whats-not-gonna-work-in-wordpress-20/