Make it easier to edit the Arduino IDE theme

This topic has come up previously in https://github.com/arduino/Arduino/issues/3056, https://github.com/arduino/Arduino/issues/3633, https://github.com/arduino/Arduino/issues/6025 and https://github.com/arduino/Arduino/issues/6438 but the latter two topics are more focused on controlling specific properties via the GUI.

This also comes up regularly on the Arduino forum and, as with https://github.com/arduino/Arduino/issues/1878, https://github.com/arduino/Arduino/issues/3056, https://github.com/arduino/Arduino/issues/3633, https://github.com/arduino/Arduino/issues/6025, https://github.com/arduino/Arduino/issues/7451, https://github.com/arduino/Arduino/issues/7780 it's almost always an accessibility issue rather than aesthetics so this is important.

I'm sure most users would prefer some sort of a GUI theme editor but I don't know how feasible that would be. This could be a standalone application or added to **File > Preferences**. Offering even the most minimal control of the theme would end up adding quite a few items to Preferences so it would probably be best to add a separate tab, as is already done with the Network settings.

A more simple, if less user friendly option, is to just document the location of the theme files and the function of the various properties. I have made a start at this in the readme of https://github.com/per1234/ThemeTest but it needs some polishing to make it more accessible to the average user. Since this is more advanced information, it might make sense to add it as a page on the [GitHub wiki](https://github.com/arduino/Arduino/wiki). The Arduino Playground is another option.

Below has been solved by https://github.com/arduino/Arduino/pull/7115:
~~A significant problem is that the theme files are stored in the Arduino IDE installation folder and thus any changes to those files will be lost after upgrading to a new version. The solution is to move these files to a persistent location such as the Arduino15 folder, as is already done with preferences.txt.~~

Start by reviewing the linked prior issues, the ThemeTest README, and the existing File > Preferences and Network settings entry points. The issue leaves the solution open between a GUI editor and documentation, so first define the chosen scope and acceptance criteria before identifying implementation files or tests.

Written by the indexing model from the issue text.