Add support for both dark and light themes

[Greenheart]

We could use the black + IDG primary colors theme from 2022-12-12 as a foundation for a dark theme.

It might me possible to move the theme out to separate config files for example, and change colors using CSS variables. OR maybe there's a tailwind specific way to solve this.

Automatic theme detection would be nice too, along with a toggle button.

A good starting point could be the branch https://github.com/Greenheart/idg.tools/tree/feature/dark-mode