Here’s how it looks after the fix:
![colors](https://github.com/user-attachments/assets/11c78ad6-da50-4aad-b133-9be5e3844878).
White is white and black is black, as intended.

In some cases, dimmed colors were poorly defined, so I took
`text.dimmed` values.

Note that white is defined exactly as the background color for light
themes. Similarly, black is the exact background color for dark themes.
I didn’t change this, but many themes intentionally make white and black
slightly different from the background color. This prevents issues where
programs assume, say, a dark background and set the foreground to white,
making text invisible. I'm not sure if we want to adjust these themes to
address this; just noting it here.


Closes #29379 

Release Notes:

- Fixed ANSI black and ANSI white colors in built-in themes