As a user, I want to be able to switch to a dark mode theme.

[mr-woodapple]

Implement a dark mode. Also make sure the website stores the preferred theme in LocalStorage.

Todo:

• Implement dark mode in CSS
• Implement toggle (navbar?) to switch between the selection
• Save the selected theme into LocalStorage
• Upon loading the website, check if there's a value for the selected theme stored in LocalStorage, load that if present

[mr-woodapple]

Currently switches themes based on the prefers-color-scheme, might be nice to include a dedicated button though.