Light/Dark mode switch

[Evernight]

It's great that the dark-mode has been added, thanks! I personally usually use it with the most websites when available. However, I'm not sure I prefer it with Fava and it also doesn't work well with fava-dashboards yet (which I use extensively). While the latter would probably be a nice fix for fava-dashboards repository, in the meantime it would be great just to have an ability to switch between the light and dark mode (which is typical for the websites that support it).

I checked whether it's possible to configure per-website config in the browser and it seems it's not common or easily available (without plugins). So the switch in Fava either via button or via config option would probably be the best solution in the meantime. Probably the button would be even better as it also would work with publicly hosted https://fava.pythonanywhere.com/.