It's easier to download themes straight from Cydia and load them. If you have downloaded them on your pc and need to install them, you either need to use a program on your pc like DiskAid to transfer to the root of your iPhone, or you need to install SSH to transfer via WiFi. I recommend the former.
you can use you usb connection of you iphone to Pc once you already download a theme. There many themes in internet that capable for you iphone just type the model of you iphone.
