Broken light theme

### Describe the bug

I suspect the black background on the user prompt is the result of a broken config. The text is very difficult to read due to the low colour contrast. The same issue affects the selection highlight, which also lacks sufficient visibility.

### Affected version

GitHub Copilot CLI 1.0.61.

### Steps to reproduce the behavior

Switch to light theme.

### Expected behavior

Have a lighter colour for highlight and for prompts, if any. I don't see why prompts need any background colour.

### Additional context

_No response_

Start by reproducing the issue in GitHub Copilot CLI 1.0.61: switch to the light theme and inspect the prompt background and selection highlight styling. Locate the light-theme color configuration or terminal rendering entry point, then verify that prompts and selections have sufficient contrast and that the light-theme behavior is readable.

Written by the indexing model from the issue text.