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.