What is the best way to set up demo for Drupal themes and live switch between them?
http://drupal.org/project/switchtheme is the one to use for that.


  Adds a block to allow users to switch between enabled themes.
  
  Themes can be given 'user friendly' names and access can be controlled
  by user role.


If you need to enable/disable blocks for certain themes, do so at Admin &gt; Structure &gt; Blocks for that theme.