Introduce the dark theme

[kayrus]

Users should be able to switch to the dark theme.
In addition modern browsers support automatic switch based on the OS setting for the dark theme.

[edda]

We would like to have this, too. But unfortunately it isn't as easy to do as you might assume as Elektra's styles weren't written with a possible theme switch in mind. Which means there would have to be a lot of refactoring done before we could even think to add a dark theme. And then of course a dark theme would have to be designed in the first place. We will hopefully do this at some point but it's not gonna happen soon unfortunately...