Add Dark Mode

[davidkrantz]

Having the possibility to enable a dark mode in the app is a highly requested feature. The user should be able to toggle a switch to enable and disable the dark mode, e.g. located in the alternatives tab under "Settings".

It should not be too much work to implement this feature since we are already using variables defined in _variable.scss for many of the colors throughout the app. It also seems like Framework7 has a Dark color theme that perhaps can be utilized.