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_

首先在 GitHub Copilot CLI 1.0.61 中复现该问题：切换到浅色主题，并检查 prompt 背景和选择高亮的样式。定位浅色主题的颜色配置或终端渲染入口，然后确认 prompt 和选择内容具有足够的对比度，并且浅色主题的显示效果清晰易读。

由索引模型根据 Issue 内容生成。