Allow customization of colors

### Describe the feature or problem you'd like to solve

More granular theme definition to allow users to set colors in a way that suits their needs/preferences.

### Proposed solution

As an example, I would like to be able to set a higher contrast background (white on blue) for user prompts, so its very easy to orient yourself to the latest context, when multitasking between many different CLI's. Being able to customize link colors, CLI background colors, text colors, etc, to allow for better readability and scanability.

Ideally there would be a way of extending /theme to support custom themes, defined somewhere in a simple theme file.

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先查看现有的 /theme 入口点以及当前主题的定义方式。确定一个简单自定义主题文件的范围，以及它必须支持的颜色类别，包括提示符、链接、背景和文本。完成标准是用户可以定义并应用具有可独立配置颜色的自定义主题。

由索引模型根据 Issue 内容生成。