How do I change the Console colour of Arduino IDE?

It turns out that the theme.txt configuration file is not located alongside the preferences.txt file, but alongside/under the Arduino application's .jar files. Thus, for the current version (1.6.3):

• on OS X, .../Arduino-1.6.3.app/Contents/Java/lib/theme/theme.txt
• on Win7, C:\Program Files\Arduino\lib\theme\theme.txt
• on Linux, .../arduino-1.6.3/lib/theme/theme.txt
• on Windows 10, when installed as an App, C:\Program Files\WindowsApps\ArduinoLLC.ArduinoIDE_1.8.10.0_x86__mdqgnx93n4wtt\lib\theme\theme.txt, however it does not seem possible to edit it - it can be opened in notepad++ but a save fails.

See an associated Github issue, IDE Background and print color options needed, for a little more discussion.