The way *this* multiblog technique works, you get (example) 20 blogs = 20 databases (or 20 tables in 1 database) = just one wp-admin folder, one wp-content folder, on wp-includes folder.
Whatever is stored in the database is unique to each multiblog. Whatever is stored in the physical files is the same for all multiblogs.
Your selected theme is a variable stored in the database -- therefore each blog can select a different theme. But all the themes must live in the 1 same wp-content/themes/ folder. Alternately -- all the blogs can use the same theme. This is what we want to do.
Now you can imagine, if you change something in the default theme (example, you change something in the CSS), the change appears on all the blogs using that theme.
Anything that is from the database is specific to each multiblog. Example, each blog displays that blog's own Blogname and Description in the header of the theme, even though they are using the same theme. Of course, this is because the theme has a variable in the code (a template tag) to display the blogname which reads from the database.
Each blog can run different plugins, because your plugin selections that are activated/deactivated are stored in the database. BUT each blog can only choose from the same set of plugins -- those that are in the master blog's wp-content/plugins/ folder.
Each blog can show different widgets.
Another interesting example is the header-color of the default (Kubrick) theme. Kubrick has its own Options panel that lets you choose different colors for the header (you can change it from the standard big blue). With 5 blogs running the default theme, each blog can choose a different header color. I assume this is because the header-color-option of the default theme is not hard-coded in the theme, but rather it is a variable established somewhere (your chosen color is saved in the database, I assume).
pixelgecko and myself, we want to use the SAME theme for all the multiblogs. Example, just pretend it's the default theme. However, we want to change 1 thing: the header graphic should be unique for each blog. For me, I just want a different picture on each blog, instead of merely a different color (which I can already do).
Therefore I think we need to call a variable from the database, which identifies which multiblog you're on, in the address of the header.php or the header.jpg.
What do you think?