Add color change option

[xuwhite]

With safari dark mode the selected colors lack readability. It would be nice to have the option to choose themes or pick different colors.

[xuwhite]

[ayman]

The should autodetect dark mode and change the colors. But I see the purple is not compliant. I'll see to updating that. Adding custom-css would be cool but would take some more doing. Open for a PR if someone wants to contribute that.

[ayman]

@xuwhite just pushed prettyJSON for Safari Version 2.2 offers a more compliant dark color pallet. I'll leave this open as a requested enhancement if someone wants to take it on (or a reminder to add full customization later).