[theme] Agents color and Dark mode (ANSI colors only)

The idea of using a background is nice, but the text color should be black when dark themes are used (e.g., dark ANSI colors only).

The same logic should apply to the foreground text color in any other places where a background color is used.

Actual: preview color of agent is not readable if used dark theme
Expected: invert or use black color for foreground text color if defined background color in UI

<img width="375" height="322" alt="Image" src="https://github.com/user-attachments/assets/9ee81450-b1c8-4739-ba9c-bcd4d5371058" />