Dark mode functionality

[AXYZE9]

• Dark mode should be activated if user prefers dark mode (ex. mediaquery)
• User should be able to toggle between light/dark mode and that preference should be saved in localStorage.

Ref: https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/

[create-issue-branch]

Branch issue-128-Dark_mode_functionality created!