This PR updates the indicator for changes within a fold to use the
correct color from the theme:

<img width="380" alt="Screenshot 2024-01-08 at 12 52 56 PM"
src="https://github.com/zed-industries/zed/assets/1486634/b8bf6bf3-6acc-43a4-8c4b-a02c975c7f02">

Release Notes:

- Updated the color of the indicator for a fold containing modified
lines.