Add support for theming

[oleast]

Is your feature request related to a problem? Please describe.
Some apps may use their own theme or use specific colors for things or sections of a page.
Examples of this would be:

• Techtalks could use the design-system, but they have their own color profile

• Things like grades.no if it ever were to be refurbished could probably have its own color profile?

• Other projects, like school projects or general use outside of Dotkom/Online.

• Adding sections/modals with negative or alternative color profiles in pages that use the regular one.

• Supporting dark mode properly (maybe even automatically).

• Supporting seasonal or personal theming.

Describe the solution you'd like
There are two good solutions to this problem that both have their drawbacks. But there may be a way to do them both at the same time?

1. --cssVariables:
We could define all our colors as CSS variables in a root element.
This solution works really well with how browsers work and will let us handle styling from back-ends and server-side rendering seamlessly.
It also has the benefit of working with media queries, like prefers-color-scheme to set colors.

Selecting a theme is done by selecting a specific global style-sheet. When changing something in the middle of a page you add an override to an element that will cascade to all children.

2. Styled Component Theme:
Styled components has a built-in theming system, where a theme is accessible as a prop in all styled components.

Selecting a theme is done by adding a by giving a different theme object to the top-level theme provider for the app. Changing the theme in the middle of a page will require a new ThemeProvider with a config to override the top-level one.