Theme should be installed in sketchbook

[knghtbrd]

On lab computers, the Arduino IDE (and its files, including the theme) are going to be read-only. Themes may be installed in your sketchbook folder as zip files. Please consider adding directions for this. (You might make it easier by moving files around in your repo as well.)

Some basic info on how it's done nowadays is here:

https://forum.arduino.cc/t/tutorial-how-to-change-the-theme-colors-in-arduino-ide/672413