[Feature Request] Add configurable UI theme (e.g. Dracula dark theme)

Summary

It would be great to have a way to change the interface theme of the OpenWorker desktop app. Right now the UI ships with a single built-in appearance and there's no Settings option to customize it. Concretely, I'd love a Dracula dark theme.

Motivation

I use OpenWorker as a coding companion, and a customizable dark theme helps with long sessions, eye strain, and consistency with the rest of my tooling (editors and terminals are all Dracula). Being able to switch the app UI to a dark scheme like Dracula (or any theme) would make it feel native to that workflow rather than a bright outlier.

Proposed options (any would help)

• A theme picker in Settings → Appearance (Light / Dark / System, plus a couple of named themes such as Dracula).
• A custom CSS / injectable stylesheet hook so users can theme the UI themselves.
• Respecting the OS dark-mode setting as a baseline.

Context

• App version: OpenWorker 0.2.1 (Windows 11 desktop build, Tauri + WebView2)
• The bundled sidecar only exposes a Pygments syntax palette ("dracula" is code highlighting, not UI), so there's no current way to tint the interface itself.
• Issue opened on behalf of a user; happy to test a build or provide more screenshots if useful.

Expected behavior

Open the app → the whole interface (sidebar, chat, panels) is rendered in a dark Dracula color scheme, or a Settings toggle lets the user choose it.