Summary: Some licenses aren't suited for code. It's recommended that BSD is used for code and Creative Commons for artwork, unless you want to add restrictions for others.
Hello modifiers and theme authors.
As stated in our Customization Approval Guidelines (http://wiki.simplemachines.org/smf/Customization_approval_guidelines#Customization_License), we require that each customization submitted specifies a license. It is our wish that our customizers embrace Open Source Software, so choosing a suitable license is vital. More information can be found in this post (http://www.simplemachines.org/community/index.php?topic=461016.0).
Our previous recommended license list included the Creative Commons (http://creativecommons.org/choose/) license. Unfortunately, this isn't suitable for code, as is explained here (https://wiki.creativecommons.org/Frequently_Asked_Questions#Can_I_apply_a_Creative_Commons_license_to_software.3F).
Although we can't fix the packages out there which already have a license, we kindly ask our Customization authors to edit their packages and re-release them under a more suitable license. Furthermore, from now on, we will require new submissions to use a suitable license.
You can find out more about software licenses here (http://wiki.simplemachines.org/smf/Customization_Licenses).