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_

首先檢視現有的 /theme 入口點，以及目前主題的定義方式。確定簡單自訂主題檔案的範圍，以及它必須支援的色彩類別，包括提示字元、連結、背景和文字。完成標準是使用者可以定義並套用具有可獨立設定色彩的自訂主題。

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