How can I create themes on my PC & not on my phone.
simple use scrappy Genymobile/scrcpy: Display and control your Android device (github.com) unfortunately no other alternatives are out there except mirroring your android on pc.
Actually it’s SCReenCoPY 
But true, best way to link your phone to your pc.
There are some alternatives/reviews here:
How to Remotely Control Your Android Phone From a PC (makeuseof.com)