What I do for multisite with same database is modify the settings.php file for an individual site.  In the $conf array you can override system variables, such as "default_theme".

Hans

Ivan Sergio Borgonovo wrote:
On Thu, 10 Dec 2009 11:44:31 -0200
"Pablo L. de Miranda" <pablolmiranda@gmail.com> wrote:

  
Hi,

I'm trying to configure 2 sites with the same installation e same
database, but when I switch the the theme of one of them the theme
change on another.
Have someone this problema too? How can I resolve this?
    

If they share the same DB *and* all the tables this behaviour is
normal.

Depending on what you'd like to share you may pick up *some* table
to share using the prefix technique.

Some articles I've found useful to build up a multisite:


Run multiple sites from the same code base (multi-site)
http://drupal.org/getting-started/6/install/multi-site

Share a single database across multiple sites
http://drupal.org/node/2622

Domain Access
http://drupal.org/project/domain

Migrating the Drupal way. Part III: managing multiple sites with
Drupal
http://acquia.com/blog/migrating-drupal-way-part-iii-managing-multiple-sites-drupal

How to best setup ubercart in a drupal multisite setting?
http://www.ubercart.org/forum/support/1456/how_best_setup_ubercart_drupal_multisite_setting

Sharing Drupal tables between databases using MySQL5 Views
http://devbee.com/node/4

Managing multisite content from a single Drupal instance
http://devbee.com/managing_multisite_content_from_