Custom theme support

Hello GH Copilot App people.
This is the first month that my copilot usage has surpassed my claude code usage.
It is because of you. This is the best app for AI I've used, and I used a few.
My suggestion is custom-theme support.
Keep up the good work.
Artur

---
| Field | Value |
| --- | --- |
| App version | 1.0.26 |
| OS | Windows 10.0.26200 |
| Theme | Espresso |
| Path | /chat |
| Tenure | Week 3 |

The issue names the desktop app and the /chat path but no files, tests, or implementation entry points. Start by locating how the app currently handles the Espresso theme and review the /chat UI. Done should mean that users can configure and apply a custom theme, with the supported scope and validation agreed before implementation.

由索引模型根据 Issue 内容生成。