MC-Class Dark Theme per ek-mc

MC-Class Dark Theme applies a high-contrast dark interface to mc-class.gr.

Features:
- Dark color palette with improved text contrast
- Styling for key layout/components (LessonBox, AccordionCard, notifications, absences, menus, etc.)
- Better pagination and dropdown readability (including Select2)
- Navbar toggler visibility fix
- Replaces default NoPhoto.jpg placeholders with an inline SVG dark placeholder (no external image dependency)

Technical notes:
- Lightweight content-script/CSS extension
- No remote code execution
- No analytics/tracking
- No external network calls added by the extension

Source code: https://github.com/ek-mc/mc-class-dark-firefox-extension