which version do you have? if it is the latest version then go to 'Tools' up there .. then 'Add-ons' then it would open a window for you. From the second tab labeled 'Themes' click on the bottom of the right side of the window where it says 'Get Themes' because i guess that you did not install anything yet. It will open the website and you can from there download themes and install them. You can later change it the same way here as well as you would be having those themes already there.

Hope that helped