I cant really tell you how to do it, it's too complicated for me to instruct you on this forum. Basically I would copy the default theme to a new folder, install it, and start playing around from there. You will need to edit the .tpl files (like header.tpl, or some of the modules theme files), the .css files, and the positions of the modules (for instance putting the search module in displayNav). I would simply do a search on the internet for "how to make a prestashop theme", you should find good info there. Thirtybees is based off of Prestashop 1.6, so the theme making/modifying is basically the same. Sorry if I couldn't give you the answer you are looking for, but a search should be a good place to start.
edit: doing the search I suggested, came up with this, it seems like a good series, from what little I watched... https://www.youtube.com/watch?v=uu4Hz08gX6U