Feature Request: Dark / Black Theme Support

Summary

Add a dark/black theme option to Dillinger for better readability in low-light environments.

Motivation

Many users prefer dark themes to reduce eye strain, especially during long writing sessions. Dark mode is now a widely expected feature in modern editors.

Proposed Solution

• Add a dark theme CSS (e.g., dark.css or via CSS variables)
• Add a toggle button in the toolbar (sun/moon icon) to switch between light and dark
• Optionally persist the preference in localStorage

Alternatives Considered

• Using prefers-color-scheme media query for automatic detection

Would love to work on this if the maintainers are open to it!