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_

Start by reviewing the existing /theme entry point and how themes are currently defined. Determine the scope of a simple custom theme file and the color categories it must support, including prompts, links, backgrounds, and text. Done means users can define and apply a custom theme with independently configurable colors.

Written by the indexing model from the issue text.