For your own use, no permission of any sort is needed.
If you want publish your reskin (and that includes sending it to friends) then you must have the permission of the original creator. This might be included in the asset, as part of the comment, for instance in the License tag. The comment must explicitly state that redistribution is allowed (possibly with conditions), or that copyright has been abandoned. Silence on the topic does not constitute approval to redistribute. Or it might be that you get permission by asking the creator (in which case you should add a comment to that effect in the asset description or license tags). No response from the creator does not constitute permission to publish. The term 'Freeware' does not constitute permission to publish. In some cases the original creator has posted a comprehensive disclaimer of copyright for distributing reskins on-line: in that case you should mention that in the asset somewhere. There are alternative forms of reskinning that do not involve publishing any content that is not yours - you could investigate those other options if necessary.
