Added Dark Mode

[Medha170]

Problem

BB-611 Adding Dark Mode

Solution

This PR introduces a dark mode theme to the BookBrainz website. The implementation includes the following:

• Added a new dark mode SCSS theme and variables.
• Implemented a toggle switch in the user interface to switch between light and dark modes.
• Updated components to support dynamic theme changes.
• Ensured that all pages and components are styled correctly in dark mode.

Areas of Impact

Before dark mode:

After adding dark mode: