It would be good to have original author's permission yes. At least that is what I require if someone is using my work.
Like Led Mirage once put it:
Do not take my work, use it to make a new skin and then do nothing to acknowledge the use of my work. While it is very flattering to see my work used throughout many other themes and many of you actually acknowledge it in your descriptions (thanks!), it is quite annoying and discouraging to find "new" skins using my work without a single mention of doing so and in some cases even bragging about all original work made from scratch, please do not do this, all it does is detract me from releasing updates more often.
In case you can't contact the original author and therefore use things without one's blessing remember to credit the original author properly