Richardm
Several things combine to form this whole theming thing:
1, subsites have these parameters that let you set where the subsite should look for the css and the whole template for every page
2. if you make your own package you want to add theming to, it can extend subsite, then you will notice it has some parameters, that look a lot like subsite parameters. Then, you would copy openacs-default-theme and point the theming parameters at your copy; editing the files inside the copy will only affect your package and its children.
3. if you want a subtree of your site to look different than the parent of that subtree, you can put a subsite there and adjust the theming parameters.
4. if you want the whole site to look different, you can either alter files in openacs-default-theme to suit, or you can make a copy of openacs-default-theme and adjust the main site's theming parameters so they point at your new theme and then edit the files in the copy.
In my case, I notice the site-node map which can tie domain names to subsites, and also notice that if I do a site that way, I might want to theme it separately, so my app extends subsite, and it has its copy of openacs-default-theme.
