I had the same issue and logged a case with IBM. Their answer was:
Yes, unfortunately customizing skins is not supported in Dashboard. This was logged for Developement in 8.4 and the workaround was to modify the user preference settings to the one of the default styles.
This was the work around a customer offered...
Create skins folders in the Tomcat 6 application templates (basically copying the corporate skins XML and folders and renaming them to our skin) in the following folders: "x:\cognos\c84\tomcat6.0.14\webapps\c84\WEB-INF\applications\dashboard\fragments\css\skins" and "x:\cognos\c84\tomcat6.0.14\webapps\c84\WEB-INF\applications\dashboard\web\skins"