Font color settings

here we go again...
using IDE 1.8.12, the font color of the comment text (the //.... ) is practically invisible and there's no setting file that has that setting.
C:\Users\spill\AppData\Local\Arduino15\preferences.txt - nope
C:\Users\spill\AppData\Local\Arduino15\lib\theme\theme.txt - wrong
C:\Program Files (x86)\Arduino\lib\preferences.txt - fat chance
and, of course, File -> preferences does no good

Very close! You were on the right track, but somehow you ended up in the wrong place on your hard drive.

The theme setting for C++-style comments (// ...) is editor.comment1.style. That setting is indeed defined in a file named theme\theme.txt. However, C:\Users\spill\AppData\Local\Arduino15\lib\ is not the right location.

By default, the theme is set by the files of the lib\theme subfolder of the Arduino IDE installation (which appears to be located at C:\Program Files (x86\Arduino on your computer).

You can modify the theme in the files at that location if you like. However, you must remember that all the changes to the theme files there will be lost every time you update to a new version of the Arduino IDE, so be careful to back them up somewhere safe. A more convenient place to put a modified theme is under the Arduino sketchbook folder . You can just put the theme folder there. The sketchbook folder is not affected by Arduino IDE updates, so the modifications to the theme will persist through updates. If you don't know the location of your sketchbook folder, you can find it in the Arduino IDE at File > Preferences > Sketchbook location.

One thing you should note is that the Arduino IDE loads the theme on startup. So you must close all Arduino IDE windows and then start the IDE again before any changes you have made to the theme files will take effect.

You can find quite a bit of documentation on the Arduino IDE theme system here