theme: Improve contrast for One Light's `read_background` token (#25027)

Closes https://github.com/zed-industries/zed/issues/20575

Subtle change, but more pronounced in single characters.

| Before | After |
|--------|--------|
| <img width="700" alt="Screenshot 2025-02-07 at 4 58 16 PM"
src="https://github.com/user-attachments/assets/dc9332e1-6e86-44a9-bb7a-5f195d778127"
/> | <img width="700" alt="Screenshot 2025-02-07 at 4 58 16 PM"
src="https://github.com/user-attachments/assets/d8fd408f-f8ea-4b87-bd20-345ffc24b5b1"
/> |

Release Notes:

- N/A