A: Your Nokia C7 warranty expired around 2012. So no. But if you hack the phone (install custom firmware) for unsigned themes, you risk bricking the device. Stick to signed .sisx files.
Disclaimer: Always backup your phone data before installing unsigned software. This guide is for educational and nostalgic purposes only. nokia c7 themes free
Free themes exist, but they must be or the phone must be hacked (patched to allow untrusted installations). A: Your Nokia C7 warranty expired around 2012