Feature/Dark mode

[vb]

Added dark mode to settings panel #54

• The value is stored both in the state and in local storage for storage and faster rendering.
• I picked the colors from @insin design in the issue
• My initial idea was to toggle a html class and use currentColor css value inheritance. But since that breaks the encapsulation (and it was way harder to use currentColor than I thought) I skipped that. There is a component in 24771e7 if anyone is interested.

[insin]

Thanks for the PR, I like having a single dark mode toggle in place - removes the temptation I was feeling to go all YAGNI while adding a single new theme 👍

I think some of the highlighting stuff I was playing about with in the example screenshot needed some content changes to support getting the blue highlights in the right place - will check when I'm back at the machine I did that on.

[catskull]

Can we get this merged sooner than later? Literally the only thing keeping me from.using this exclusively.

[addyosmani]

Friendly ping @insin in case he has a chance to review (it seemed like there were content adjustments necessary for the right highlights) 🐐

[catskull]

Bueller...