My arduino foreground text is a gray on a white background. I looked on line and tried changing theme.txt, but can't get the text foreground color to go to black.
Sorry this doesn't directly answer your question, but I prefer this theme which is high contrast and inverted color (light text on dark background, easier on the eyes): https://github.com/jeffThompson/DarkArduinoTheme
Again, doesn't specifically answer your question, but I feel it helps readability significantly.