How to manage multisite themes from master Drupal installation
The README.txt file in Drupal's /themes directory states:


  …the sites/your_site_name/themes directory
  pattern may be used to restrict themes to a specific site instance.


For instance, you might have:

- sites/all/themes/m1
- sites/childsite1.com/themes/c1
- sites/childsite2.com/themes/c2
- sites/childsite3.com/themes/c3