Many Drupal 6 users have reported that they could no longer control styles after upgrading the Panels module in their Drupal installation. If you are facing the same issue in your Drupal site then read on to find out the solution.
Currently the only solution is to patch the Panels module with the following patch. http://drupal.org/files/1412158-administer-panels-styles-d6.patch or wait for a new release with the commited patch
Hope that helps.
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