Add dark mode

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Hello!

First of all, thank you for this very useful extension — it has helped me a lot given the situation in my country, and I’m really grateful for your work.

As a small contribution (and to be easier on the eyes), I added dark mode support to the options page:

• Automatically detects the system color scheme (light/dark) on first load
• Adds a theme toggle button to switch between light and dark mode
• Saves the selected theme in chrome.storage.local and restores it on future visits
• Listens for OS theme changes and updates automatically when the user hasn’t manually chosen a theme

Thanks again!