Adds dark mode

[A-Guldborg]

This PR aims to add dark-mode, since we are getting compiler warnings that the Palette is deprecated and we should change to PaletteLight (+ PaletteDark).

This uses the system default preference on first render, but if the user actively changes the setting it is supposed to stay changed (currently, this restarts on full refresh / navigate to page anew but not on navigating through the site - see subtasks).

Most of the colors are based on what @marfavi liked, though in my opinion the dark mode is a little too dark...? But please help me with inputs on this :) I have attempted to find a good text-color that can be used instead.
I am also considering "bundling" same colors together (such that we don't define the textcolor three different places to be the same value).

Subtasks

This PR is a draft and is missing two main features:

• Using localstorage to save user preference if they actively change it
• Figure out a way to handle "deactivated" menuitems/products (Please come with feedback/ideas on how we can handle this, e.g. a global darkmode variable, only changing non-color related stylings or similar)