By default, folder owners can apply a different style to the folders that they own. However, if you want to enforce a consistent look across the whole content area, you need to revoke this privilege, so that folder owners cannot affect the style of their folders.
Revoking this privilege also means that users with Edit Style privileges on a folder cannot apply a new style to that folder.
If you revoke this privilege, folder owners and users with Edit Style privileges can still create new styles. However they cannot apply those styles to their folders.
You must be the content area administrator, the portal administrator, or have Manage privileges on All Content Areas to revoke folder style privileges from folder owners.