Add Dark mode

[Nerajno]

• [x] Implement color scheme switcher
• [x] Use CSS variables for theming
• [ ] Persist user preference

[Nerajno]

See also #18 Is notes :tAdd dark mode, considerations :

• [ ] https://www.kevinzunigacuellar.com/blog/dark-mode-in-astro/
• [ ] Toggle switch to navbar
• [ ] Accessible Astro Component

[Nerajno]

Got it to work thus, now I need to figure out or laydown the CSS variables well.