Credit goes to reddit user PrettyMuchPhysics

Convert the book to its existing format in Calibre. To do so, right-click it, select convert, convert individually. Make sure that on the top left and top right, both formats are the same (e.g. EPUB).

1. Then on the left select "Look and Feel".
2. On the top row, select "Styling",
3. Put a checkmark at the bottom right option where it says "Color".
4. Press OK.
5. After the book finished converting, right-click it and send it to the device, as usual.

This made the font an easy-to-read black, instead of a light-gray nightmare.

https://web.archive.org/web/20230608085927/https://old.reddit.com/r/kindle/comments/io79ru/solved_gray_font_in_a_book/