Add dark mode

[mjkl-gh]

I've added a line that looks up a user's preferred mode and sets it according to browser preferences.

To make it work I had to remove the custom background color (#fafafa), since it would be set after the dark or light theme was selected. Resulting in a dark mode with a white background.

The current background color is now (#ffffff)

I will try and see if I can re-incorporate the custom background-color

[mjkl-gh]

Just added another commit reincorporating the custom background color for light mode