As on line shopping is been a famous trend now so people are also earning through Themes, but doing so they are not working hard or I better should say are cheating with their work. In this way they only are deceiving themselves.
With the passage of time cyber world is now too much crowded and to stand out of this crowd one has to be come with uniqueness which is only possible with originality.
Now for example there are many Magento themes available on internet, and as on line shopping trend is under your observation so you can think about the demand of shopping carts. Now one who is think to establish an on line store will when search for templates then surely will try that his/her site should be the most attractive. Hence, he/she will select the theme which he/she found as unique one on cyber world. Now those who have copied other's themes will get no use of their site.
It is also possible that the selected theme by that person is also copied by somewhere. But in that case most probably only the idea has been copied, which is not a wrong approach. So if you are impressed by any design then you can present that idea but should be in original and new way.