CSS string + instructions to change notes' background to dark theme

[zazench]

This should be able to resolve issue #17

Instructions on how to set up the string in Zotero can be found in file: set_note_color_to_dark.md

The string itself is note.color.dark.css

Full credit for the original css snippet to Zotero Forum user fbcx. The string can be found in this Zotero forum topic and this comment . I just edited it to match with this theme.