Same filename in this branch
  1. 6.x themes/README.txt
  2. 6.x sites/all/README.txt
  3. 6.x modules/README.txt
Same filename and directory in other branches
  1. 10 themes/README.txt
  2. 7.x themes/README.txt
  3. 8.9.x themes/README.txt
  4. 9 themes/README.txt
This directory is reserved for core theme files. Custom or contributed
themes should be placed in their own subdirectory of the sites/all/themes
directory. For multisite installations, they can also be placed in a subdirectory
under /sites/{sitename}/themes/, where {sitename} is the name of your site
(e.g., www.example.com). This will allow you to more easily update Drupal core files.

For more details, see: http://drupal.org/node/176043

File

themes/README.txt
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  1. This directory is reserved for core theme files. Custom or contributed

  2. themes should be placed in their own subdirectory of the sites/all/themes

  3. directory. For multisite installations, they can also be placed in a subdirectory

  4. under /sites/{sitename}/themes/, where {sitename} is the name of your site

  5. (e.g., www.example.com). This will allow you to more easily update Drupal core files.

  6. 

  7. For more details, see: http://drupal.org/node/176043

  8.