Add dark mode

[tsafran]

Added a switch for toggling dark mode in settings under the skip password switch. Currently only primary and primaryDark colors are changed upon toggling because I wasn't sure about the color pallete. To customise the dark mode look just change color values in the colors.xml(night) file.

[CLAassistant]

All committers have signed the CLA.

[rorymbyrne]

Thanks for this. It was a feature that many people requested. I will ask @Coccodrillo to have a quick check out it, the merge :)