Thank you for contacting us in our forums.
Yes and No actually, first is Yes, it is possible to make your own theme and customize it fully according to your own desire.
But No, because there is no easy way on doing this because you cannot just upload a pre-coded or pre-designed theme and say that it's done.
You can go to your Admin Panel, then look for "THEME BUILDER", with the theme builder you can slice the images from that theme and upload it manually and design your own website. We have lots of customers that are using iClan Websites that at the same time, they are the one who designed their own theme. You can also watch the tutorial video that you can find on the Theme Builder itself.
Just let us know if you need any further assistance.