Add dark/light theme toggle to the orb UI

## Context
The orb visualizer currently has a fixed color scheme. A dark/light theme toggle would improve usability in different environments.

## What to do
Add a theme toggle button to the orb UI. The orb is built with Vite + vanilla JS/HTML.

## Files to modify
- `orb/src/` (UI components)
- `orb/index.html` (toggle button)

## Difficulty: Easy
Standard CSS theme switching.